Shout out for Sandi Metz book POODR, and her Railsconf talk The Magic Tricks of Testing, if you're a rubyist (though the principles hold true for non-ruby OO programmers too).
+1 for POODR - very (very) well written, goes down multiple pathways reasonably (rather than "this is how you solve that" without any clue why you solve it that way), and gives some decent tools for any project. I only wish it were longer.