Check the library at www.pmillett.com. Also visit boozhound labs and read the writeup on how to build a tube amp. Enlightening.
If you want to buy books, Bruce Rozenblit and Morgan Jones have some excellent ones.
More than anything, try some DIY. Build a CMoy and then move onto another project or several. Nothing teaches you how stuff works like building it and reading how each part functions. You'll absorb so much more with hands-on work. When marketing claims turn up, you'll be able to decide whether they're real or a bunch of nonsense. You'll also learn what stuff actually costs, so overpriced gear will be very obvious. You'll be able to tell good construction from garbage.
Not just amps, but building a DAC is a great project, and so are speakers.
You'll not only understand the gear, but you'll be able to see through a lot of audio nonsense. And there's a lot of nonsense. There's a very good reason why 99% of audio gear isn't marketed towards those handy with a soldering iron and capable of reading schematics. So skip all the stuff that doesn't present a technical analysis. Yes, there's a learning curve. However, it is fun to learn, it'll keep you from getting ripped off, and you'll be able to build excellent gear for much, much less than commercial gear. There are only a few manufacturers I'll buy from. The rest either haven't a clue or are desperately overpriced. Sometimes both.
