Testing Business Ideas, by David J. Bland and Alexander Osterwalder, is a business book. I read The Lean Startup earlier this year, which talks a lot about the need to validate your ideas (you know, before you bet your whole company on your assumptions).
I knew a few ways myself, but this book goes deep into the various ways, how useful the evidence from them is, the best ways to do them and how they connect, including examples in some cases. Usually several pages worth per method, including some very useful ideas on the best way to implement them.
There are classics, such as customer interview, expert stakeholder interviews, e-mail campaigns, online ads, link tracking, customer support analysis, and many many more.