covington. It's much too crude for serious work given the alternatives. (See the linguistics tag for some ideas of alternatives (click on the "learn more" link)). If your glossing needs are extensive then you should look into ExPex; but minimally something like
linguex is better for linguistic examples. Also, you might want to consider using XeLaTeX which will allow you to use IPA fonts directly in your source, which certainly aids readability, although perhaps with slower input.
As for templates, for book length documents, it's probably best to go with one of the more comprehensive document classes such as
memoir or the KOMA classes. Book design itself is an art, not a science, so if you find some other grammar or language learning book that you like, you can probably replicate its design with one of those classes and a few choice packages. See What are the strengths and weaknesses of KOMA-Script and memoir? for an overview of some of the differences between the two classes.
You also added the ebook tag. This is a whole different issue, which is perhaps best addressed in a separate question. See Use LaTeX to produce Epub for some ideas.