just on the chance that somebody might find this thread while looking up induction heating, or maybe the OP drops back in for another look, i found a pair of books on induction heating that was written for training classes for foundry workers. the books are in the same pictorial/ informational format that was used for many military electronics training courses during WWII. the books are very descriptive of induction heating precesses, and even teach the basic math needed for understanding induction heating. in volume 2 of the texts, a lot of good information is laid out on types of induction heating equipment, including motor-generator, vacuum/gas tube power oscillators, spark gap oscillators, etc... basically volume 1 is mainly about the physics and basics of induction heating, and volume 2 is more about the electronics, mechanics, and equipment. i had these books when i was a teenager. when this subject came up in this forum, i remembered those books, but i couldn't find them right away online.
https://catalog.hathitrust.org/Record/007471770
one thing i found interesting in volume 2 was the arrangement of a power oscillator tank circuit with a tank coil and a work coil. also the description of how the tank circuit works as part of the heater coil circuit. the RF current coming into the tank circuit is 300A for instance, but the circulating current at resonance (with the workpiece in the coil) in the tank circuit is around 1400A.