Yes, Your Perfect Right is a classic and is regularly updated. It is a good choice especially if you're looking for a book to recommend to clients. Alberti & Emmons also published The professional edition of 'Your Perfect Right': A manual for assertiveness trainers (which I haven't read). It sounds like a good choice if you're looking for a guide to assertion training.

If you're looking for book that reviews theory and research on assertion and assertion training, I'd recommend Assertive Behavior: Theory, Research, and Training by Richard Rakos (Routledge, 1991).