This book is a good introduction to the Lua Programming language (v5.1), and assumes you already know how to program in another language. It starts with the basics and goes on to quite an advanced level, but at all times it's clear and easy to understand. There are plenty of examples throughout. There aren't many books on Lua, so this one should definitely be on your bookshelf if you want to program in Lua 5.1.
There's quite a lot about how Lua can interact with C, so to get the most out of it you need to know C already. But if you don't know C and just want to use Lua as a scripting language, you'll still benefit from this book.
If you're interested in calling Lua from a C program (easy), or if you want to extend Lua's capabilities by writing your own C functions (harder), then this book will show you how to do it.