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[[The Lua ebook]] Reference Manual
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The official definition of the Lua language is its reference manual, which is available online and also as a book:
Lua 5.1 Reference Manual
by R. Ierusalimschy, L. H. de Figueiredo, W. Celes
Lua.org, August 2006
ISBN 85-903798-3-3
Also available in Portuguese , Spanish , and Russian .
The manuals for all released versions are available online.
Link:
http://www.lua.org/manual/5.1/This is totally free, but may be a bit complex to beginners. This is what you use if you already know what a "boolean" is.
Also available (but outdated by .1 versions)
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This is an online version of the first edition of the book
Programming in Lua
by Roberto Ierusalimschy
Lua.org, December 2003
ISBN 85-903798-1-7
The book is a detailed and authoritative introduction to all aspects of Lua programming, by Lua's chief architect. The first edition was aimed at Lua 5.0.
Link:
http://www.lua.org/pil/