There is some excellent news, posted by Richard Zach. (Although, I imagine most of my readers will have seen this on his blog. I think this information should be spread as much as possible. It is a Very Good Thing.) The Association of Symbolic Logic has decided to release several out of print logic books for free through Project Euclid. These include books from the Perspectives in Logic series and the Lectures Notes in Logic series. They are available in chapter-by-chapter .pdf. Be careful about downloading a lot at once or your IP will get banned. As Richard says,

This includes classics like

It is an excellent selection of books. The Barwise volumes, in particular, are gems that are nigh impossible to obtain elsewhere. (HT: Richard Zach)