A humble suggestion for Roberto (or anybody else): a thorough guide to Lua as extension and extending Lua.
I just finally paid good $$ for PIL 4, and am glad I did. However, it's main focus, naturally, is Lua programming. The material on the C interface is good as far as it goes, but it leaves me hungry for more.
I would pay good Bitcoin for a Guide to writing Lua bindings that covers the hairy bits - like dealing with coroutines, strategies for organizing code, etc. - in a little more detail, with examples, maybe covering multiple languages.
There's plenty of info along these lines on the web, but it sure would be nice to have an authoritative Guide.
My 2 cents,