Here is my top 25. In no particular order (though most of them are series).
1. The Count St. Germain Series by Chelsea Quinn Yarbro
Yarbro's series is about the Count St Germain, who in her series is a vampire, and his adventures throughout history. The original five are Hotel Transylvania, The Place, Blood Games, Path of the Eclipse, and Tempting Fate. Other stand outs in the series, imo, include Blood Roses, Better in the Dark, and the three Olivia novels. The series can be read out of order, and the historical period tends to be different per book.
2. The Sunne in Splendour by Sharon Kay Penman
Moonight is completely right about Penman, and this is book is just great. If you have read Daughter of Time and not this one, something is wrong with you.
3. The Egyptian historical books by Pauline Gedge
Gedge has written other historical than her ancient Egypt books, but they are the best. There are two trilogies' (The King's Man and Lord of Two Lands), a duology (House of) and three stand alones. She has a book about the Roman conquest of Britain.
4. Benjamin January Series by Barbara Hambly
Hambly's long lasting mystery series follows Ben, a freed slave who has trained as a doctor in Paris. He returns to New Orleans shortly after the purchase where he is only seen as a piano player.
5.The Accursed Kings by Maurice Droun (at least 1-3)
Apparently this series inspired George R R Martin. It traces the fall of the Valois dynasty.
6.The True Story of Hansel and Gretel by Louise Murphy
It isn't really the true Hansel and Gretel, but the fairy tale set in WWII Poland. The best part is what Murphy does with the Stepmother and witch characters.
7. Robert Lawson's Ben and Me as well as Mr Revere and I
Lawson wrote quite a few children's books about animals. Ben and Me is about Ben Franklin told by a mouse, and Mr Revere and I is the famous ride retold by the mare that Revere rode.
8. The Eleanor of Aquitane novels by Elizabeth Chidwick
It's true that Chadwick seems a bit centered on William the Marshall, but her books about Eleanor are superior.
9. Joplin's Ghost by Tananrive Due
This is one of those alternate time novels. Part of the novel concerns Scott Joplin, the other part the girl who has his cursed piano. Its really, really good.
10. A Place of Greater Safety by Hilary Mantel
This doorstopper is set during the French Revolution/Terror. You can smell Paris.
11. Wildflowers of Terezin by Robert Elmer
This book is set in Nazi occupied Copenhagen and is about a pastor and a Jewish woman making it though the war and falling in over.
12. Code Name Verity by Elizabeth Wein
Wein's book is told by two young women who are part of the British SOE group sent into occupied France.
13. The Three Musketeers by Dumas
I mean, is there a book that has been filmed more?
14. The Light Bearer by Donna Gillespie
The story of a girl from the Germanic tribes as they resist Rome. The ending fight in the arena is kick ass.
15. The Collaborators by Reginald Hill
This novel follows the lives of several people in Occupied Paris.
16. An Instance at the Fingerpost by Iain Pear
A mystery told in four different points of view.
17. Fingersmith by Sarah Waters
Takes the gothic, adds the history and lesbians.
18. The Hound and the Falcon by Judith Tarr
Tarr's series is set during the Crusades and follows a young priest who may be able to do magic.
19. The Terror by Dan Simmons
The basis for the AMC series. The novel reveals what really happened to the Franklin expedition to find the Northwest passage. It is a slow book, but that's part of the point.
20. The Lambs of London by Peter Ackroyd
Ackroyd's book imagines what might have happened if Charles and Mary Lamb (she killed their mother in a fit of maddness) had met William-Henry Ireland (the men behind the Shakespeare fraud).
21. Cassandra Princess of Troy by Hilary Bailey
Epic retelling of Troy.
22. Segu by Maryse Conde
Set in Africa as Islam and transatlantic slavery invade, the novel chronciles the lives of a family.
23. Autobiography of Henry VIII by Margaret George
George's novel set some critics off because she has Henry contend that Kat of A was lying. But it is told in Henry's pov, so well, duh. If anyone captured his voice, she did. I just people would stop putting the book in the history section.
24. The Memory Man by Lisa Appignanesi
One of the best books about WW II.
25. David Ashton's McLevy
This series started as series on BBC radio. It is based on the real McLevy who patrolled Edinburgh during the reign of Queen Victoria and published his memoirs. In addition to the radio programs, there are a series of books. Well worth a read.