This is an excellent reference book. Explains the operation and theory of all the different type of audio amplifiers (solid state), with schematics etc. as well as typical uses for power transistors. Can help a lot when troubleshooting transistor amps.
https://ia600900.us.archive.org/22/items/RcaPowerTransistorApplicationsManual/RcaPowerTransistorApplicationsManual.pdf
https://ia600900.us.archive.org/22/items/RcaPowerTransistorApplicationsManual/RcaPowerTransistorApplicationsManual.pdf