In 1959, the Athlone Press of the University of London sought to publish a handbook to Old Church Slavonic in its London East European Series. Grigore Nandris wrote the first part of this handbook, a grammar of the language. Later, Robert Auty published a selection of readings as the second part.
Nandris covers the writing systems (Cyrillic and Glagolitic), phonology, morphology and word formation of OCS. His approach is diachronic, as he discusses the sound changes that separate Common Slavonic from other branches of Indo-European, and he traces noun and verb inflection back to Proto-Indo-European.
I've collected several introductions to OCS, but Nandris' is my favourite. There's something really special about his work. His presentation of paradigms is clear and accessible throughout. The Athlone Press has typeset his manuscript beautifully and the book is easy to navigate. All OCS words are presented in Cyrillic. The description of the OCS verb (including aspectual distinctions) is especially exhaustive.
There are some omissions here. I do not consider them faults in the book, but readers should be aware of them since they may have different needs. Unlike some other presentations of Old Church Slavonic, Nandris does not draw comparisons between OCS and other early Slavonic languages. While the book has a comparative approach in that it refers to Proto-Indo-European, it does not talk very much of how OCS and the South Slavonic language it was based on relates to the West and East Slavonic branches. And a further consequence of this is that Nandris does not discuss the prosody of OCS at all, as accent and tone is not discernible from the OCS texts themselves (there is no agreed-upon interpretation of the markings in the Kyiv Folia) but only through comparative Slavonic linguistics, and that's what Nandris chooses to avoid. Finally, Nandris does not dedicate any space to syntax, but since the texts are almost entirely translations from Greek that is understandable.
If you are looking for a friendly introduction to OCS grammar, Nandris' book is a fine starting point, just make sure you get a printing from 1965 or later that incorporates corrections. As far as the competition goes: Schmalstieg's grammar is detailed and has a comparative approach, with much reference to Baltic, but the book is badly typeset and cluttered. Lunt's book is purely synchronic, which at least for my purposes makes it useless. Gardiner's book is little more than an outline, really a waste of money.
(Verified purchase from a local antiquary.)