The Wayland book is IMO the best starting point at understanding how Wayland works.
It explains all the concepts of wayland from the ground up, which is extremely valuable. My main problem with it is that it is only set toward Wayland clients, and the compositor side is considered "out of scope". It's unfortunate because that was the part I was mostly interested in, which was really frustrating. I'm however glad that I read it, because even if it's client oriented, it goes a long way at explaining the main concepts.

Such a shame it was never finished… But given how modular the Wayland protocol is, it is hard to explain concepts out of the main interfaces (eg, clipboard, XDG shells, …).

Definitely worth a read for anyone digging into Wayland !