1. Diana: Her True Story — In Her Own Words by Andrew Morton

2. The Crown: The Official Companions by Robert Lacey
If you're looking for fictional accounts of British monarchs of the past...
4. The Henriad Tetralogy (Richard II, Henry IV Part 1, Henry IV Part 2, and Henry V) by William Shakespeare

5. Fatal Throne: The Wives of Henry VIII Tell All by M. T. Anderson, Candace Fleming, Lisa Ann Sandell, Jennifer Donnelly, Linda Sue Park, Deborah Hopkinson, and Stephanie Hemphill

6. My Lady Jane by Cynthia Hand, Brodi Ashton, and Jodi Meadows

If you're looking for some fantasy that shows the dark side of royalty and power...
7. Cinderella is Dead by Kalynn Bayron

8. Forest of a Thousand Lanterns by Julie C. Dao

And if you need a break from the period drama and want a book with romantic pining or Princess Diaries feels...
9. Her Royal Highness by Rachel Hawkins

10. How to Catch a Queen by Alyssa Cole

11. Tokyo Ever After by Emiko Jean

And for a book written by a cast member...
12. Shiverton Hall by Emerald Fennell
