Here's an introductory logic textbook in pdf format called 'For All X', written by a State University of New York professor. It is a little peculiar in my opinion because it addresses formal semantics/model theory before it addresses proofs. It seems to me to be comprehensible to intelligent newcomers to the subject.
http://www.fecundity.com/codex/forallx.pdf
And here's one out of Cambridge University entitled 'Teach Yourself Logic'. This one seems to be an introduction to logic for new graduate students in philosophy who didn't study the subject during their undergraduate years. So it addresses the introductory material from a more sophisticated viewpoint, I guess, showing how the advanced topics develop out of it. It's more of an outline and a study-guide than a text, and it covers a lot of material very quickly. This one has lots of bibliographic reading suggestions for follow-on reading in various topics covered, so in a way it's a guide to the undergraduate-level logic literature.
http://www.logicmatters.net/resources/pd...ic2016.pdf
And here's a more advanced text to be read after one has the material in 'For All X' under one's belt, suitable for a second logic course perhaps. It emphasizes material of interest to computer science, such as computability and Turing machines, but is suitable for philosophers as well. This one is called the 'Open Logic Textbook' and it comes out of the U. of Calgary.
http://people.ucalgary.ca/~rzach/static/...mplete.pdf