First off, study electronics. Get a book that covers all the basics, and then start on amp-building books.
Well, the kind of books you want to get depends on what you want to do really. If you want to start designing amps, you will need an extensive knowledge of electronics, and how to apply it on guitar amplification. If you want to build an amp from a pre-designed scheme or something, a basic knowledge of electronics, and decent soldering skills should be enough.
Before starting on amps, try building your own stomp-boxes though. They'll cost a lot less if/when you make a mistake and fry them. And, they are a lot safer to be experimenting with too.
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