That's specific enough that I doubt there is an introductory book specifically. You should just read current (or classic) research articles about it.
However, for Kratzer's approach to Formal Semantics in general-- including modals-- there are some textbooks that will get you started, if that's what you want.
Three that come to mind (used in classes I've taken or taught):
Kate Kearns (2000 or 2011 2nd ed.):
Semantics, publ. by Palgrave. -- an accessible undergraduate level introduction to Formal Semantics broadly compatible with Kratzer's work with some basics of modal logic
Irene Heim & Angelika Kratzer (1998):
Semantics in Generative Grammar. -- A similar but more detailed and challenging graduate-level introduction to Formal Semantics. A good place to start but modals are only mentioned in passing. After this background, see the followup textbook for modals specifically:
Kai von Fintel & Irene Heim's draft textbook (2011):
Intensional Semantics. (As far as I know still unpublished but accessible online and useful.) It picks up where the Heim & Krazter textbook ends and goes into more depth with mostly 'intensional' topics, including a full chapter on modality (with reference to Krazter's work and a short but helpful bibliography).
http://web.mit.edu/fintel/fintel-heim-intensional.pdfhttp://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.229.37So it depends on how "introductory" you want, but I think those textbooks would be at least a good place to start. Beyond that, I'd go right to Krazter's research articles (the bibliography at the end of the chapter in the third textbook might get you started). You could also check some handbooks/encyclopedias for overview chapters.
