And for a book by Paul Rand, I would recommend A Designer's Art if you can find it.

http://www.amazon.com/Paul-Rand-A-Designer%60s-Art/dp/030008...

I asked myself the same question around 6 months ago, so I talked to the most talented designer I know and asked him for a list of the most influential books he had read whilst developing his skills.

Here is the list, in the order he recommended reading them:

Creative Process: http://www.amazon.com/Made-Stick-Ideas-Survive-Others/dp/140...

Design Thinking: http://www.amazon.com/Paul-Rand-A-Designer%60s-Art/dp/030008...

Typography: http://www.amazon.com/Thinking-Type-Designers-Critical-Stude...

Spatial Relationships - Grids: http://www.amazon.com/Making-Breaking-Grid-Graphic-Workshop/...

Reduction: http://www.amazon.com/Symbol-Steven-Bateman/dp/1856697274/

Color Theory: http://www.amazon.com/The-Art-Color-Subjective- Experience/dp/0471289280/

Branding 101: http://www.amazon.com/Designing-Brand-Identity-Essential-Bra...

I've been making my way through the list, and it is amazing how much my approach to design has changed. Before I was winging it and was never really sure if I was making something that would look good, but now I feel like I have a solid foundation to build on.