There's a discussion on the Clojure mailing list about how to learn to "think in Clojure" (or think in Lisp or, really, think in functional programming terms). A prominent recommendation is The Joy Of Clojure by Michael Fogus and Chris Houser, which everyone says is a great book, but here are a couple of free online books that were also recommended:
- Structure and Interpretation of Computer Programs by Abelson, Sussman, and Sussman. It's the "entry-level subject in computer science at the Massachusetts Institute of Technology" and it uses a dialect of Lisp called Scheme, not Clojure, but it provides a good grounding in both computer science and functional programming.
- How to Design Programs by Felleisen, Findler, Flatt and Krishnamurthi. This is another introduction to programming / computer science style book that also uses Scheme for its examples.