I enjoy accurate historical description as well as proper grammatical language. Obviously we might be unable to read old or middle english, but what irks me at times is finding slang terms are being used that didn't exist in medieval times. I'm not talking about any author in particular, just something I have noticed from time to time.
There are alot of great authors out there who write accurately of history and customs. If you can track any of these down, it would be very rewarding read, I promise!
Susan Wiggs's The Lily and the Leopard (King Henry V)
Joan Wolf's Road to Avalon (King Arthur)
Roberta Gellis (Roselynde series)
Anya Seton
Jean Plaidy (AKA Victoria Holt)