It doesn't appear that either of those books will teach you Lua specific to ROBLOX. They are essentially the same book, except that the 3rd edition will have been updated. The description says
This Third Edition updates the text to Lua 5.2 and brings substantial new material. In particular, it brings more than one hundred exercises distributed through all chapters, ranging from simple questions about the language to full small-size projects.
That's the difference between the two books.
[Edit]
The first edition of that book is available for free online. I highly recommend it. Another great resource is the online Lua interpreter available here. It will allow you to try out some of the things you are reading about. Good luck to you in your quest! It is an exciting adventure!
I propose using Lualearners mumble, they have helped me get through some rough scripts -> www.lualearners.org.