Programming in Lua by Robert Ierusalimschy is the authorative book about the language. The third edition of the book has exercises at the end of each chapter, this repository contains my solutions to these exercises and will be updated as I read the book. There is one folder for each chapter in the book, in the format <chapter number><chapter name>. I also try to add examples of the code/syntax introduced in each chapter (in the examples folder of each chapter) and some notes (in notes.txt).
There is an online version of this book, however I do advise you to buy it if you want to learn the language, both to support the author and since a paper copy is advantageous when learning. If you do not need a paper copy you can still but the ebook version.