Packed with a tape machine I got recently was a book. The title is "Tape Recorder Service Manual And Trouble-Shooting Workbook" by Robert Marshall published by Chilton (#980) in 1962. Though old and focusing primarily on tube electronics and mechanical transports with idlers and belts etc., it gives a great and pretty detailed look into how tape recording works in general as well as a lot of basic electronic theory for the beginner. The main idea was to equip the tape recorder user of the day with knowledge to understand how their machine worked, logical repair and troubleshooting procedures. I guess the best thing about it is that it gives a lot of information about electronics, particularly old electronics as it existed 1962 all in one place. If you are a beginner restoring a machine of that era this book and Phil Van Praag's "Evolution of the Audio Recorder" are both helpful.
Some sample chapter headings:
The Electronic Components in an Audio Device
The Anatomy of Your Tape Recorder
Electrical Circuits in a Tape Recorder
Trouble Symptoms
etc.
Well worth looking for as a resource even if it knows nothing of microprocessor controlled logic and barely acknowledges these new transistor thingies.