Learn plain Lua first or just jump into Luau?

Agree on Luau. The one bit worth going deep on is tables, metatables especially. Most beginner Roblox tutorials skim them, but that's what OOP-style modules are built on, and once they click a lot of other people's code suddenly makes sense.

Skip the book and go straight to Luau. It's built on Lua 5.1, so the basics are the same anyway: tables, functions, loops, scope. What a standard Lua book teaches that doesn't carry over is mostly file IO, the os library, loading modules from disk, and the newer 5.3/5.4 features. Time spent on things Studio doesn't even let you use. Luau also adds stuff plain Lua doesn't have, like += , continue, string interpolation and type annotations, which you'll want anyway. the official creator docs on scripting plus making tiny things in Studio will get you further than any book.