Eleanor Hibbert wrote under several different pseudonyms but her most famous was probably Victoria Holt. The following is a list of her books. I don't know if it's complete but it is a list of what I've read and some I have not read but am familiar with the title. Most are Gothics but some are historical romances.

1. Mistress of Mellyn
2. Kirkland Revels
3. Bride of Pendorric
4. The Legend of the Seventh Virgin
5. Menfreya in the Morning
6. The King of the Castle
7. The Queen's Confession
8. The Shivering Sands
9. The Secret Woman
10. The Shadow of the Lynx
11. On the Night of the Seventh Moon
12. The Curse of the Kings
13. The House of a Thousand Lanterns
14. Lord of the Far Island
15. Pride of the Peacock
16. The Devil on Horseback
17. My Enemy the Queen
18. The Spring of the Tiger
19. The Mask of the Enchantress
20. The Judas Kiss
21. The Demon Lover
22. The Time of the Hunter's Moon
23. The Landower Legacy
24. The Road to Paradise Island
25. Secret For a Nightingale
26. The Silk Vendetta
27. The India Fan
28. The Captive
29. Snare of Serpents
30. Daughter of Deceit
31. Seven For a Secret
32. The Black Opal

At some point, I'd like to go back and write a quick synopsis of each book and my review on them, so I'd be able to keep track of which is which.

Those are a lot of books. I don't recall reading any of them at the moment, but I might have. It would take a while to read all of them. Why did she use different names?

Beats me. Most of the books above are Gothics but a few are historical romances involving real historical figures. I read a few that she wrote under the name "Philippa Carr" and they appeared to be historical romances with fictional characters, although some may have been classified as Gothic. I never read any under "Jean Plaidy" but they appeared to be historical books about historical figures. I don't know if they are fictional accounts or factual bios.

Victoria Holt was used for the Gothic books she wrote. Other books under other names were primarily historical romances. Since her output was so great, it was thought that the use of different names would aid the public in finding the books they wanted. Barbara Michaels/Elizabeth Peters is a similar story. It's just a way of organizing an author's work when she delves into more than one genre.