Cessna,
Read this thread - there's a lot on information on here about what's entailed, and once you apply, NATS will send you everything you need to prepare yourself for the Stage 1 tests - not really any books available nor needed.
May I ask why you're considering NATS now after strongly pursuing a career as a Flight Instructor? If you do get past the first few stages, I'd expect some hard questioning from HR about your change in career...