Hi David! To me, 'grammar reference book' conjures up either a grammar terms dictionary like Chalker & Weiner's
Oxford Dictionary of English Grammar, or an actual
grammar (i.e. a description of English from morpheme to potentially discourse level, but focussing mostly on the clause if not sentence, and the variety to be found therein), which may vary in size and depth, from books such as Eastwood's
Oxford Guide to English Grammar or the
Collins COBUILD English Grammar through to veritable tomes such as Quirk et al's
CGEL, Biber et al's
LGSWE, or Huddleston & Pullum's
CGEL.
'Usage book' meanwhile conjures up usually lexical (i.e. word-based, A-Z "quick entry" format) descriptions that are as factual, non-technical and bare-bones as possible. Representative examples would be
Collins COBUILD English Usage, or Swan's
Practical English Usage.
Lastly, 'grammar practice books' are well-represented by Murphy's
Grammar in Use series.
But of course there are other varieties of, or ways to describe "grammar books" (e.g. 'pedagogical grammars' is a term that could cover all but the "tome-y" of the above titles), and what a more experienced teacher might sniff at as a mere 'grammar practice book, more suited to students than teachers', might well be a detailed, indispensible "reference" to a novice teacher (i.e. inexperienced teachers are often advised, for better or worse, to begin with the likes of Murphy, inadequate though that might be in developing a detailed enough understanding to really teach or indeed simply
understand grammar well enough (to perhaps
not teach certain aspects of it over or before other aspects)).
Over the years I've actually posted a fair bit about grammar books. Here are two example threads:
http://forums.eslcafe.com/teacher/viewt ... 1198#41198 (you'll need to scroll down through the post a bit)
http://forums.eslcafe.com/job/viewtopic.php?t=71476
Hope this helps.
BTW, 'Elementary Ed' is more about educating young learners than teachers themselves. (Just wonderin' why you posted this question here
). Don't be scared of joining in on the Applied Linguistics forum, for example!
http://forums.eslcafe.com/teacher/viewforum.php?f=3