It depends on what interests you. For the War of the Wales, I recommend each of their respective sides:
Morton for her and
Dimbleby for him. One of the more balanced books would be
Diana in Search of Herself by Sally Bedell Smith.
I would also recommend reading the books written by those who have worked for members of the RF.
Paul Burrell:
A Royal Duty
P.D. Jephson:
Shadows of a Princess
Ken Wharfe:
Diana Closely Guarded Secret
Simone Simmons:
Diana The Last Word
I would however, take these four books with a grain of salt. They are each written for specific reasons which are not exactly the right reasons. Wharfe's book is probably the most balanced of the four. Jephson's is filled with bitterness but does offer a great deal of insight into her life. Simmons and Burrell both vie for the glory of being the most important figure in Diana's life which ends up sounding more pathetic than important.
The better books written by insiders are:
Mary Clarke:
Little Girl Lost (she was Diana's nanny)
Wendy Barry:
The Housekeepers's Diary
Marion Crawford:
The Little Princesses (QEII and Margaret's nanny)
Stephen P. Barry:
Royal Sevice (former valet to Prince Charles)
Malcolm J. Barker:
Courting Disaster. This book is really very funny. However it focus' on the behind the scenes of the Royal household and the people who work for the RF
If your looking for some older history try:
Matriarch by Anne Edwards. It's about Queen Mary and one of the best I have ever read on her.
Alexandra by E.E.P Tisdall. On Queen Alexandra and again on the of the best I have ever read.
I highly recommend reading the Duke and Duchess of Windors' autobiographies/memoirs. If you can find them, they are wonderful. (
The Heart Has it's Reasons is hers and
A King's Story is his).
The Duchess of Windsor The Uncommon Life of Wallis Simpson by Greg King is by far the best written, best researched and most balanced I have read on them.
All these books you can usually find at discount books stores or online at discounted prices. Good luck!