Author: Stephen D. Krashen
First published in 1982. That is getting close to 30 years ago. I'd love to read something more recent and see how his ideas have changed or evolved.
He distinguishes between language learning and language acquisition. Language learning is a conscious process of grammar study, vocabulary memorization and drills. Language acquisition is a subconscious process of internalizing comprehensible input.
Comprehensible input is really the big thing I took away from this book. He theorizes that we do not need to spend our time practicing grammar and memorizing vocabulary, and that these things are not really helpful to the process of speaking a language most of the time. He believes that is is the quantity of comprehensible input that determines our language ability.