These are "old" references, as a lot of Forth references seem to be but I enjoyed them during a Forth phase. The standard advice, and I agree with it, is write your own Forth, maybe more than one. If you're comfortable in any platform's assembly language I would recommend starting there, even if it's "old". This depends on your own history in computing but I found a C64 emulator convenient for this. Old PC emulators have load/save abilities to the host file system, and what I loved was a simple memory model, memory-mapped hardware features, and a simple ISA. i.e. a system you can hold in your head all at once, contrasted with modern handhelds or desktops.

Two books: (I really enjoyed the first one, multiple times)

"Threaded Interpretive Languages", by R. G. Loeliger.

"Stack Computers: The New Wave", by Philip Koopman.

Two short papers: (maybe not profound, but they're short and available online)

"Handling Multiple Data Types in Forth", by John J Wavrik. https://www.math.ucsd.edu/~jwavrik/pub/11_jfar.pdf

"Literate Programming in Forth", by Peter Knaggs. https://rigwit.co.uk/papers/literate.html