The engines are, indeed, unusual. The ones discussed are primarily British, secondarily European. No surprise, considering the nationality of the authors.
The Junker Jumo and Deltic engines get relatively little mention. They are, comparatively, conventional.
Swashplates instead of crankshafts. Multiple crankshafts, geared together. Free-piston engines, where the gas exhaust spins a turbine. Opposed piston engines of every configuration you can imagine; and a few that I hadn't imagined. Rotating valve trains. Engines for cars, trucks, airplanes and tanks. Racing engines. Nautical engines. Motorcycle engines.
Rotary, radial aircraft engines where the intake comes through the crankcase and through a valve embedded in the piston crown.
You know, unusual engines.
I learned quite a bit about what was possible in engine design by learning about what had been tried and what had/hadn't worked and why.