TITLE: What books to use to start studying Mathematical Logic?
QUESTION [8 upvotes]: I want to study Mathematical Logic. One concept that confuses me, is that implication is equivalent to '-P or Q'. So, I want to start from the book where this idea first started; but I'm not looking only for this idea, but also other basic ideas of Mathematical Logic.
I guess Boole's Boolean Algebra helped build Mathematical Logic. Can you give a brief explanation of how it and other ideas did (Like the previously mentioned implication definition), where they first started (in which books), and what other classic books talk about them?

REPLY [3 votes]: "Mathematical Logic", these days, connotes a quite advanced study (it is often the title of third-level logic courses in universities, for example). What the OP means actually wants, I guess, is a lower-level introduction to formal logic (a.k.a. symbolic logic). 
There are a lot of good books out there (and quite a lot of not-quite-so-good ones too!). @amWhy mentions a good one that is now freely available. You'll find a few more suggestions at various levels in the introduction to the Teach Yourself Logic study guide which you can download from http://logicmatters.net/students/tyl/