I have posted this before, but the topic came up in the shoutbox and I think this is very important. Many people want to design PCBs for their projects, which is great. But some of the ones I have seen are terrible and likely to not work at all. The book linked here is fairly short, easy to read, and contains a wealth of great tips and techniques, plus an overall philosophy of how to design a great PCB. I consider it a "must read" for anyone doing a PCB design.