Book title: Simple and Usable
Author: Giles Colborne
A book about interaction design - basically the art of adding usability by removing fluff and unused/unusable features. The book was originally targeted at web and mobile app development, but I find the principles therein as useful for API design (for instance). Learning to understand your user applies just as much to the user of your software, whether that user is an end-user/operator or a developer using your software components.
/Steen
CLA, CTA, CLED & LabVIEW Champion