There are many, many books written about Princess Diana.  I've read lots of them!
Here are personal reviews of some of my favourites.
For each one I've given a "crown star rating" from 1-5. 
Most are available at amazon.com or amazon.ca .  Enjoy reading about our favourite Princess!
Author's Note:  These views represent my personal opinion only.

The Housekeeper's Diary
Charles and Diana before the breakup
by Wendy Berry
Shadows of a Princess
An Intimate Account by her Private Secretary
by P.D. Jephson
This is an easy read written by the Wale's Highrove housekeeper.  Set in the Wales' weekend retreat Highrove during the tumultous years leading up to the separation of the Prince and Princess of Wales.  Banned in Britain, this is a true insider view of what happened as the marriage irretrievably broke down.  My son gave this to me for Mother's Day many years ago and it's a favourite for that reason alone!
This is a robust and rich account of Princess Diana's "business" life.  Written by Patrick Jephson,  Diana's private
secretary  for 8 years, this is a must-read for anyone that wants to know a different side of this complex Princess.  A terrific read if not always a flattering portrayal of Diana.  You may be surprised!
The Diana I Knew
Loving Memories of the Friendship Between and American Mother and Her Son's Nanny who Became the Princess of Wales
by Mary Robertson
A Royal Duty
by Paul Burrell
A loving account of the Princess by her "rock" and butler Paul Burrell.  This story details the insider view of Paul during his time at Highgrove and then Kensington Palace after the breakup.  Ever loyal to the Princess, you'll really get  to see the vulnerable side of this incredible woman as she battled to juggle children, royal duties, a crumbling marriage and an adoring but demanding public.  Paul also shares some great insights about the Queen from his time as a royal footman.  A classic!
Mary Robertson employed Lady Diana Spencer to nanny her baby son in early 80's London.  This story tells the tale of the teenage Diana before and during her courtship to the Prince of Wales.  A great first-hand account of a specfic time in Princess Diana's life.  Perhaps a little too much information about the author's life than the average Diana-fan cares to read.
Diana, Her True Story
in her own words
by Andrew Morton
Ever After
Diana and the Life She Led
by Anne Edwards
The explosive best-seller and most famous book about Princess Diana is a must-read for any Diana fans.  Originally the Princess denied any knowledge of this book but it later came out that she had cooperated with the author and others to "tell her story" to the world.  The closest thing to an autobiography that we'll ever have - it's packed with shocking revelations about the famous Princess unhappy in her castles.  Perhaps not as accurate as Diana would have liked us to belive, it's nonetheless a riveting read. Followed up with Diana, In Pursuit of Love detailing the last five years of her life. 
I really liked this biography of the Princess.  It strives to be neutral although firmly in Diana's camp.  Although it's clearly well researched, the author does not quote or attribute sources formally.  There is a lot of rich detail about Princess Diana's life, activities and relationship with Prince Charles.  It ranges from Diana's childhood through to her death.  One of my favourites!
Diana
Story of a Princess
by Tim Clayton and Phil Craig
The Diana Chronicles
by Tina Brown
Well-researched with multiple quotes by specific individuals close to the Prince and Princess of Wales, this is a pretty unbiased view of the events of Diana's life.  Written from an American perspective, it relies more heavily on quotes than description.  The book is engaging and full of insider perspectives.  Not a lot of new insights are provided but still a rewarding read for any Diana fans.
If you've never read a biography of Princess Diana, this is a good all-around account of her life from birth to her tragic death.  It tries to be neutral to both Charles and Diana and succeeds for the most part.  All the juicy goods are here from the affairs, bulimia, separation and details of Diana's death.  There are not a lot of new revelations here but the basic history is well covered.  Surprisingly there are no photographs.  Overall a solid read.
The Year of the Princess
by Gordon Honeycombe
Diana
by Sarah Bradford
I really liked this biogaphy.  Very thorough - it spans Diana's entire lifetime.  Accurate and loaded with details, this is a pretty balanced view of the Princess' life and times.  Sarah Bradford is an acclaimed royal biographer and takes a pretty balanced view of the War of the Wales.  I would have liked a little more inside information about Prince Charles and Camilla but overall this is a great read and very on the mark.    
This is a lovely but hard to find book written in 1982 - just after Prince William was born.  It documents in photographs Diana's first year of royal life.  It touches on fashion, her daily life, the wedding, etc.  You can find some lesser known facts about the beautiful Princess Diana as well as some rare photos.  Gordon Honeycombe worked with renowned photographer Tim Graham to create a visually stunning and sweet point-in-time view of the lovely Diana.
Diana in Search of Herself

by Sally Bedell Smith
Diana, The Untold Story
by Richard Kay & Geoffrey Levy
Co-written by Daily Mail journalist Richard Kay and Geoffrey Levy in 1998, this book summarizes extremely well the highlights of Princess Diana's life.  With the benefit of time, there's not much new news here but it's very well written and factually  backed-up with on and off the record insider confirmations.  It's aIso beautifully illustrated with some some photos I'd never seen before.  A great coffee table book and conversation starter.  Richard was personally close to the Princess so I was hoping for more off-the-record insights but it's still a great read.
I had a really hard time with this one.  Extremely well-researched, it's almost an encyclopedia of the life and times of Princess Diana.  Presented as an unbiased biography, I objected to the assertions of serious mental illness for Diana.  Bedell Smith claims to present a balanced view of the facts but conistently ties Diana's behavior to mental issues such as borderline personality disorder and bi-polar tendencies.  As flawed as Princess Diana was as a person, I don't agree that she was able to cope and succeeed for many years with such serious mental problems.  I guess you'll have to read it and decide for yourself!
Diana:  Closely Guarded Secret

by Ken Wharfe with Robert Jobson
The Royals
by Kitty Kelley
This is a tell-all biography by the famous Kitty Kelley (Oprah).  It reports intimate details about all the royals - from King George VI through all the current senior royal family.  There is lots of Diana facts to satisfy eager Diana fans although most sources are anonymous.  Written before her death, the focus is mostly on the Princess' disintegrating marriage and the house of Windsor's response to it.  At times a little sensational, it's still a juicy read for anyone looking for more insider info on what goes on in the House of Windsor.
Covering six years of Princess Diana's life before, during and after the breakup, this bio penned by Diana's PPO (Personal Protection Officer) Ken Wharfe is a solid read.  At times giving different versions of familiar and events and loaded with details about her frame of mind, moods and urge to spread her wings, Ken's book is highly readable and a breath of fresh air from stale accounts of the same old news stories.  One is always suspect of a book written after the author has fallen out with the subject yet it has a ring of truth throughout.  I quite liked it.
Diana V. Charles
by James Whitaker
Diana's Boys
William and Harry and the mother they loved

by Christopher Andersen
Penned by famous royal reporter James Whitaker - this insider view by a British journalist was written in 1993 - after the separation but before the divorce.  Whitaker followed Diana from the very beginning and gives a pretty balanced view of the events from the wedding through the troubled marriage to the ultimate breakup.  An interesting read to see some of the more famous events (e.g. skiing vacations) from someone who was actually there.  And if you want to read the complete transcripts of the Squidgy and Camillagate tapes - this is for you.  Overall well written and informative.
I found this book a bit of a mixed bag.  The first few chapters cover old ground (Diana's childhood, early relationship with Prince Charles, etc.) with nothing new to say.  However, the later chapters after Diana's death give a lot of great insights into the boy's childhoods, relationship with Charles, growth into young adulthood, etc.  Andersen is a popular biographist so it's a well-written book.  Personally, it made me miss Diana more!  Her legacy is very strong in her children and you can see much evidence that her loving role as a mother has had a lasting affect on these two fine Princes.  
The Truth
The Friends of Diana, Princess of Wales, Tell Their Stories
by Judy Wade
The Windsor Knot
Charles, Camilla and the Legacy of Diana
by Christopher Wilson
There's not a lot of new tidbits in this biography supposedly supported by friends' stories.  There are a few details about life in Kensington Palace contributed by former staff and/or friends.  For some reason, there is a lot of coverage of the Wales' various overseas trips to Australia.  Princess Diana's post-Charles life and relationship with Hasnat Khan are explored.  A well-written book but none of the Princess' closest friends appear to have collaborated.  A little short on "the truth."
This biography tells the story of Charles and Diana's breakup from the perspective of the other woman - Camilla Parker Bowles.  There's not much new information here and there's a lot of background about Camilla that may not be of interest to Princess Diana fans.  A good overview of the known facts about the breakdown of the relationship that was "crowded with three people in it."  Not one of my favourites.
Diana in Private
The Princess Nobody Knew
by Lady Colin Campbell
Diana
A Princess and Her Troubled Marriage
by Nicholas Davies
This is an interesting viewpoint from a well-known aristrocrat - Lady Colin Campbell.  Written in 1992 before the royal split, it was considered explosive at the time with its revelations of the Princess's bulimia and hints of infidelity on both sides of the Wales marriage.  Definitely pro-Charles, this version takes aim at Diana's moods, motives and manipulations while ocassionally giving a nod to her tough life and impact as a humanitarian.  Many courtiers went on and off the record to give their first-hand accounts.  Interesting but not as well-balanced as some other biographies.
Written in 1992, this "point-in-time" biography is definitely written from a pro-Charles prospective.  No mention is made of Princess Diana's bulimia, although there are a couple of obscure references to anorexia.  Easy-to-read and interesting from a historical perspective, it's a pretty comprehensive view of the first ten years of married royal life, just before the breakdown.  There are also a few less-famous photos along with some classics.