What's the best explanation of FSE that you've found?

[jdmarshall]

I came here hoping for a description of the algorithm. Do you folks have any recommendations for reading materials?

[Bulat-Ziganshin]

among other things, check https://shargs.github.io/data-compression/2022/07/28/entropy9.html (also part 4)

and of course https://encode.su/threads/2078-List-of-Asymmetric-Numeral-Systems-implementations :)