The various Old Church Slavonic primers that have been written over the past hundred years are of varying quality. Among the worst for this student of historical linguistics are Lunt’s Old Church Slavonic Grammar, which is purely synchronic and written from the viewpoint of structual linguistics, and Gardiner’s Old Church Slavonic : An Elementary Grammar, which is also synchronic but is objectionable mostly for looking like a set of lecture notes hastily thrown together. Things get better with Schmalstieg’s Introduction to Old Church Slavic Grammar, which is diachronic and quite informative, although Schmalstieg has his idiosyncracies (such as a bit too overtly sparring with his enemies on the page) and eschewing any mention of laryngeals.
However, there are two works that I would hold up as the best there is in the world of OCS primers. The first is Handbook of Old Church Slavonic Grammar by Grigore Nandriş (London: The Athlone Press, 1956). I don’t think I’ve ever seen a better introduction to a minor Indo-European language (i.e. one other than Latin, Greek, or Sanskrit). The logical presentation of the language’s development from Proto-Indo-European, the smart organization, and the way he seems to anticipate any questions the reader might have are delightful. In fact, I’m a bit reluctant to recommend it because I am still looking for my own copy and don’t want competition. There is a companion volume of reading selections and a glossary compiled by Robert Auty that is, for whatever reason, much easier to find on the used market and will prove enlightening to any student.
The other great primer is K.A. Vojlova’s Старославянский язык (Moscow: Drofa, 2003), which is very inexpensive and can be had from the largest bookstores in Moscow. Although a Russian-language work, which obviously assumes some prior study of Russian, its presentation of the language is very gentle and the book is approachable for students with only the slightest prior study of comparative Indo-European linguistics. It includes abundant reading selections, and was published recently enough to discuss the Eninskij Apostol manuscript which was discovered too late for Auty’s volume. The inexpensiveness of the book compared with its high quality raises questions about why Western scholarly educational materials are often so very expensive.