Lua: Resources
Lua Home The official home of Lua.
Programming in Lua, 1st ed., (online and free) This is considered the book when it comes to Lua. The first edition is still largely relevant and is a great place to start.
Programming in Lua Book, 4th ed. The forth edition is the latest version and is available in hard copy and ebook format. I purchased this book and found it a great benefit (it also supports the language authors).
Lua 5.4 Reference Manual (online and free) This is the official definition of the Lua language. Like all language reference manuals, it can be somewhat dry, but between the first edition introductory book above and this reference manual you have, for free, everything you need to understand Lua (of course, coupled with digging in and solving some problems).
Lua in Neovim Neovim (my editor of choice) utilizes Lua as the scripting language and the documentation is great. Understanding and writing Neovim plugins was a great way to improve my Lua skills.
Lua Scripting Udemy Course A thorough introduction to Lua scripting. I found it to be very useful, especially in the beginning. Wait for Udemy sales that seem to come around frequently.