Wikinomics - How mass collaboration changes everything
If you work in marketing, and haven't picked this up yet, do.
I like the authors' explanation of 'online' which is "no longer about idly surfing and passively reading, listening, or watching. It's about peering, sharing, socializing, collaborating, and, most of all, creating within loosely connected communities".
The hardback is just under 300 pages but it is very possible to dip in and out of it.