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

The Brodie books are both excellent.

"Object Oriented Forth" by Dick Pountain is a succinct and practical exploration of applying Forth's metaprogramming capabilities to extend the language with various features. The best aspect of the book is that it works through many alternatives rather than taking a direct path. It's more about data structures and abstraction than OOP.

Julian Noble (https://www.taygeta.com/fsl/docs/NobleArchive.html) also wrote a number of very interesting articles and books about Forth. There's an apparently ongoing project to re-typeset his "Scientific Forth": https://github.com/Josefg/Scientific_FORTH.