As it happens, I recently had to update the reading list for the logic paper of the first-year of the Cambridge Philosophy Tripos. One of the topic-headings is "Necessity" (see foot of p. 8 to top of p.10). That's a modest introductory list, concentrating on the notions of logical necessity and analyticity.
Unfortunately, however, this might not be a terribly helpful response, as access to most of the references given will involve you using a university library. Perhaps others will know of useful and reliable free online resources of an introductory kind (and I'd be glad to hear of them to add to the reading list too!).