Good question. Actually I don’t have much of an idea of the ‘canon’ of FL teaching books but I’m sure that you and other posters could come up with some good and useful suggestions, and if you do I’ll amend the title of the thread accordingly, or maybe a moderator could split it off.
The thing that gripped me about Krashen was how his theories described and explained so well what was going on in my classroom. The learning/acquisition distinction explained how students could reproduce language perfectly under controlled practice conditions, but would seem to have lost all the learned language by the next class. It explained the seeming limitations of error correction. It explained how some students who seemed inept at formal language learning would nevertheless aquire some language and attain some communicative ability, especially if I worked with their intuitive and creative senses.
I’m still in an internal debate with Krashen. While much of his theory is compelling and persuasive, I have yet to come to a firm conclusion, and there are some areas which it does not seem to cover adequately. It certainly merits serious thought, which is what I’m trying to give it.