One for me was Herrington's Code Generation in Action. Thinking about generating code algorithmically and efficiently changed my perspective a bit.
in reply to Which non-Perl books made you a better (?:Perl )?Programmer?
Past that, I found OS-specific books to be a big help -- Windows XP Under the Hood for example. Little tips and tricks that you can leverage in your code can make a difference.
Once a programmer reaches the stage where they question the code in a book and look for more efficient and/or secure ways to perform the same task, they are usually well on the path.
There is no substitute for experience, of course, but getting people beyond the stage of "do it like this, for this is the way I was taught" is usally a big step. TIMTOWTDI is just an acronymn until you practice it.
Just my dos centavos.