Tubebooks.org has quite a few vintage audio books available for download as PDF files. Most of these are books published before 1964 that never had their copyrights renewed and have thus passed into the public domain.

Here is a link to the Audio (hi-fi, amplifiers, speakers...) section of the site.

I have been reading "Amplifiers (The Why and How of Good Amplification)" by G.A. Briggs, who offers simple, to-the-point explanations of things like amplifier quality (distortion, noise, etc). Briggs approaches these topics from a practical rather than theoretical perspective, making it an easy read for laymen.

Briggs' book "A to Z in Audio" is also a very good concise reference.