"The non-designer's design book" is really great, I learned a lot from it. It helps to know these things, even though I am a software developer and normally do little to no design work for my clients.

Currently I read "Visual Language for Designers" (-) which is also very interesting. It describes how our brain processes visual information and how you can use this to create better (easier to understand) designs.

(-) http://www.amazon.com/Visual-Language-Designers-Principles-U...