Alison Weir is a historian that wrote two excellent fictional novels about English monarchs - Innocent Traitor, about Lady Jane Grey (who was queen of England for 9 days), and The Lady Elizabeth, which is actually about Elizabeth's life before she became monarch but is still exceptional.
I haven't tried them out yet, but Elizabeth Chadwick, Sharon Kay Penman, and Jean Plaidy are the top names in historical fiction about monarchs. Chadwick and Penman, I believe, do the Middle Ages, and Plaidy is more Tudor era. I have heard nothing but good things, ever, about Sharon Kay Penman. I've got books of hers on order right now.
Philippa Gregory is kind of tawdry and not very historically accurate, but I'd still recommend, very strongly, The Constant Princess, about Catherine of Aragon. It takes place, however, while she is princess and dowager princess, before she became queen of England.
I think you'll find that books where the main character is the monarch, rather than a subordinate figure like a lady-in-waiting, mistress, duke, etc., OR about the early life of a monarch before they came into their rule, are pretty rare, unless it's set in the period of one of their crises (for example, Mary Queen of Scots' imprisonment, Queen Elizabeth's war with Spain, Queen Mary Tudor's... entire reign). The character with all the power usually isn't the best protagonist. There are exceptions, though, like Margaret George's novel, An Autobiography of Henry VIII.
Hope this helps!