See http://www.londonpower.com/pop.htm for Kevin O'Connor's "Principles of Power".
I also second the above recommendation of Van Valkenburg's "Basic Electricity" if you are new to electronics. Older editions of his "Basic Electronics" (1950's, 1960's) also have good and easy chapters on tubes.
I also second the above recommendation of Van Valkenburg's "Basic Electricity" if you are new to electronics. Older editions of his "Basic Electronics" (1950's, 1960's) also have good and easy chapters on tubes.