If you're interested in understanding, designing, or building great-sounding amplifiers and speaker systems, "Design and Construction of Tube Guitar Amplifiers" and "Design and Construction of Guitar and Bass Speaker Systems" contain the information you need.
Both books provide easy-to-understand yet thorough descriptions of the technologies, components, and construction and testing methods used in tube guitar amplifiers and guitar and bass speaker systems.
You can find descriptions of both books, as well as ordering and contact information, by selecting the appropriate link.