A typical modern score for Haydn has a boatload of editorial suggestions for dynamics, staccato, tenuto, etc. I'm sure these editors know more than I do (that's not hard), but I'd like to know more about the basis of this. After "An Essay on the True Art of Playing Keyboard Instruments", what other reading is out there?
I can recommend "Performance Practices in Classic Piano Music" by Sandra Rosenblum. This book deals with how piano music was played in the classical period--Mozart, Haydn, early Beethoven, Clementi, etc. The author has synthesized treatises from that era with illustrations from the music, and given her own opinions on various problems of interpretation. I found the book very useful in giving me some background in how to make my own decisions on how to play a particular passage or piece.