If you're looking for a free introductory logic textbook, look no further!
forall x is a collaboratively written open educational resource (OER). It is not just freely available, but also freely customizable to your needs if you're a logic instructor.
You can download a pdf-file of the latest version that I am using here at UConn by clicking here (or the link below the preview on the left). The UConn version contains a part on non-classical logics at the end (featuring intuitionistic logic and some three-valued logics), which I contributed. (I also gently revised most of the other chapters to suit our teaching and learning needs.)
If you would like to have the LaTeX source of this version, please feel free to email me. (GitHub repository coming soon.)
To find out more about forall x, visit the forall x page on the Open Logic Project website. There are many other versions of forall x available there, including chapters on different topics (like, e.g., a few introductory chapters on Modal Logic), as well as translations into other (natural) languages.