I’m scheduled to teach a course on modal logic in the Fall. So I’ll have to think about a textbook choice pretty soon. Last time I’ve used Fitting and Mendelsohn’s First-order Modal Logic (Kluwer, 1999), which I quite like. It’s accessible, which is important, since many of the students will be philosophy majors with little formal background beyond an introductory logic course. Fitting and Mendelsohn include a bunch of philosophical material, and, as the title indicates, focus on first-order logics. So you get into a lot of interesting material about possibilism versus actualism, you get to discuss abstraction (which is important for intensional semantics), and also non-alethic logics (deontic, epistemic, time), although not in as much depth as I’d like. Proofs and metatheory are done via trees. But it’s not terribly interesting for computer science students, who also make up a good chunk of the audience. So something like Blackburn, de Rijke, and Venema’s Modal Logic (Cambridge, 2002) would be more interesting to them (and me, I guess), but is probably too technical. Chellas’s Modal Logic (Cambridge, 1980) is nice, but probably also too technical, a little outdated, and doesn’t cover first-order systems. Hughes and Cresswell’s New Introduction to Modal Logic (Routledge, 1996), like Chellas’s book, does everything Hilbert-style (my students might kill me if I make them do exercises in that), and doesn’t have a lot of discussion of applications (philosophical or computer science). Plus, I prefer boxes and diamonds to L and M. Beall and van Fraassen’s Possibilities and Paradox (Oxford, 2003) and Priest’s Introduction to Non-Classical Logic (Cambridge, 2001) don’t cover enough modal logic for my purposes (e.g., no first-order logic). Girle’s Modal Logic and Philosophy (McGill-Queens, 2000) might be interesting, but I haven’t looked at it. He does discuss many applications of modal logic in philosophy, but I don’t know how in-depth. Anyone used it? Other ideas?
UPDATE: Two more textbooks worth noting: Chagrov and Zakharyaschev’s Modal Logic, and Bell, DeVidi and Solomon, Logical Options. As I said in the comments, the former may be somewhat expensive. The latter is not only affordable, but covers non-standard logics more broadly (second order, modal, intuitionistic, and many-valued logics).