Found Kernighan and Plauger's Elements of Programming Style useful - interesting to see how many of their comments on Fortran and PL/I code still remain very valid problems.

Their further book Software Tools in Pascal I found inspiring (along with SICP) as they show how you can build very powerful and useful tools from small modules slotted together - SICP showed me this in the extreme with chapter 2 ending with an implementation of a full numeric tower