>>10194036This is a very standard intro book. It's a little verbose but definitely a good place to start if you don't have any university level math background.
If you already have some university level math background and just want to quickly cover the main ideas then Simpson's Mathematical Logic notes aren't a bad place to look.
http://www.personal.psu.edu/t20/notes/logic.pdfA few of the finer distinctions are glossed over so if you really want to get into logic it might still be better to read through Bergmanns - The Logic Book.
>>10193883Forall X is part of the Open Logic Project which means it's licensed Creative Commons and you can find it online for free. There's also a few remixes of the material available and I think the one from my university is the latest.
http://forallx.openlogicproject.org/The Open Logic Project also has it's own textbook called the Open Logic Book called the Open Logic Text. The book is also broken down into several subbooks that contain a few collections of chapters (eg. a subbook that covers modal logic).
http://builds.openlogicproject.org/Personally I wouldn't recommend this book for a beginner. It's geared more for people taking their second logic course and beyond. That said I know the main author (and I've taken a course with him using the book) I would recommend it for people interested in continuing their study of logic. The book is constantly getting chapters added to it as well (the main author writes chapters for the classes he's currently teaching).
