[j_m_b]

Niels Henrik Abel, when asked how he so quickly became proficient in mathematics said that he accomplished it "by studying the masters, not their pupils." I've learned languages by reading the writings of the author(s) of a language. Reading books is good, reading papers is even better. I didn't have a good foundational understanding of Lisp until I read McCarthy's paper on it.

I second the practical approach of working on projects. I've learned so much just by trying to do things I wanted to do, but didn't know how to do at the time.