Why does using keywords or symbols as functions to lookup values from maps work?

Question

Quoting from Joy of Clojure, section 4.3.1--

Because keywords are self-evaluating and provide fast equality checks, they\'re almost always us

Answer 1

Keywords implement IFn,

https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Keyword.java

and its invoke method handles calling get.