Recursion: SICP and later The Little Schemer.
Abstraction barriers: SICP
Continuations: The Little Schemer
Decision Trees: Uni. of. Washington MOOC about classification and retrieval and later implementing them myself in Racket and later GNU Guile.
Can't agree more. The Little Schemer took me from "getting it" at a conceptual level to really knowing how to use recursion to get stuff done. Super important for grokking data structures imo.
