Although it seems to be targeted at corporate negotiations, "No" is full of great tips for dealing with clients of any size. It's definitely changed the way I deal with clients. Where I used to try to focus on selling our services, I now write out a list of key questions and let the client do the talking. I gain far more insight into the project if I let them break it down slowly and stick to asking questions.
Camp also talks a lot about "need". It's easy to start thinking "If we land this contract I can buy a new car/hire another employee/rent an office" and get yourself set on the idea before you really figure out what's involved, or more importantly whether they are people you see yourself enjoying working with. He repeatedly stresses the importance of remaining focused exclusively on the logistics of the deal and ensure that you fully understand what's at stake before getting pushed into something you can't back out of.
Definitely worth checking out for anyone who runs a business.