As the other reviewers note, this book is remarkable in that it both offers a reasonably gentle introduction to the subject even for those without too much mathematical background (logically-inclined philosophy students come to mind), and at the same time remains to this day an indispensable reference for the specialistâ€"this for example is the only source where you can find the proofs of Solovay's "other" modal completeness results for transitive models of set theory. George Boolos was (is) famous for, among other things, his gift of didactically brilliant exposition.

There is only one other book on the subjectâ€"C.Smory'nski's "Self-Reference and Modal Logic", now out-of-print, which provides a nice complement to Boolos with surprisingly small overlap. For an introduction, I would not hesitate to recommend Boolos over Smory'nski.

Since 1993 the "baton" has not completely "fallen out of Boolos' hands"â€"the interested reader can follow up on this via e.g. a couple of handbook-chapter expositions, easily found by googling around, that point to current research literature. However, no further textbook-level exposition appears to be forthcoming anytime soon.

As a comment to galloamericanus' review, I would note that Boolos does not restrict his monograph exclusively or even primarily to his own results. The reader gets a fair overview of the research field by 1993 as shaped by many contributors.