I too welcome someone else's advice of such a book, but I often scan around for aviation management books and I cannot recall anything on this topic. There is also not much else around, as Direct-Law said.
Other sources could be articles in the USA magazine Business & Commercial Aviation. I got 165 articles searching on "flying schools" [hint - use that as well as 'flight schools']:
http://www.aviationweek.com/aw/search/BasicSearch.jsp
Most of these are not relevant, but there could be a nugget of gold somewhere there.
There are plenty of Google hits on "flying school business plan", although I am not sure if there are really useful or are rather someone marketing a generic business plan that fits all sizes from candy store to airlines.
In fact the lack of publications suggests that the topic could be a worthwhile one indeed.
