'The Medici Seal' by Teresa Breslin surprisingly enough features the Medici family, although they're not really main characters. It does have bonus Leonardo da Vinci though, and I remember enjoying it a lot when I read it.

Frieda's biography of Catherine De Medici is definitely one I'll recommend :)

Susan Carroll's The Dark Queen series has Catherine de' Medici as the principal villain. These books may not be what you're looking for, though, since they don't really focus on Catherine...plus they are romances with fantasy elements! I thought they were some fun brain candy though.

I don't have a good read on how historically accurate these might be, but they're great reads. I'm recommending "The Stars Dispose" & "The Stars Compel" by Michaela Roessner. These may be filed under general fiction, maybe under science fiction. Beautifully written, they feature a young man who is the son of a family who has long served in the Medici kitchens. In addition to several significant interactions with the young Catherine, he winds up apprenticed to Michaelanglo as a sculptor. Likely to leave you feeling hungry, these are the first two novels in a trilogy that unfortunately looks unlikely to be completed.