Via “Richard Zach”:http://www.ucalgary.ca/~rzach/logblog/2007/11/csli-lecture-notes-online-and-free.html, I see that CSLI has posted full PDFs of “many of their books”:http://standish.stanford.edu/bin/search/simple/process?query=pbk for free download. There are a lot of good things here. I basically learned modal logic from Goldblatt’s “Logics of Time and Computation”:http://standish.stanford.edu/bin/object?00003782 (warning: Large PDF), and I might well teach from it now that it is easily available.