A New Introduction to Modal Logic
M.J. Cresswell, G.E. Hughes
This is the best introduction to modal logic for all but very experienced logic students. However, the first printing (ISBN 0-415-12599-5 (hbk), 0-415-12600-2 (pbk)) has many annoying errors, especially in the exercises. Make sure to get a later printing. Right now I see only the very expensive hardbound edition, and only the error-loaded first printing, on the Amazon website.
