Discovering the REPL
I've know about REPLs for a few years, but it's the mini REPL tutorial at the end of Chapter 3 in The Joy of Clojure that showed me how useful they can be for not just experimenting but implementing something real. The REPL is an interpretative scratch-pad, and payground where feedback is immediate instead of, as with enterprise Java, painfully slow. It's an exploratory studio, providing access to the wider world of Java and Clojure.
I could have followed a different tutorial and not found this appreciation. But Fogus and Houser grow the code as it might grow for someone who is actually experimenting. I'm certain I wouldn't have accomplished what they did with such ease but the principal holds: build something in the REPL, refine, progress.