I have taught Reformation for nearly twenty years, at both university and seminary, but my philosophy of how to choose a textbook has not varied from the secular to the sacred setting: I want a book which provides good background data, empirical information and clearly laid out arguments with which I can interact. Whether I agree or disagree with the latter is of no relevance to me: the textbook is a tool. Plus, I take the view that if my classes merely reiterate what the textbook says or argues, then students would be better off just reading the book. Cheaper in terms of both time and money.
The two basic books I recommend are: Euan Cameron, The European Reformation; and Carter Lindberg, The European Reformations. Neither book is cheap but both are jam packed with helpful information, the products of two very learned and thoughtful reformation scholars at the top of their game. Of the two, Lindberg is probably of more interest to most Ref21 readers, as it has more theology. The title is also very helpful, highlighting at the outset the fact that what we typically think of as a singular event was actually a collection of different, albeit connected, happenings. It serves as my primary class textbook. Cameron, however, covers all those areas which I want to assume in my lectures -- literacy rates, economic data, social history etc -- but am not interested in pursuing in class in any great detail.
Some years ago, I switched Lindberg for Diarmaid MacCulloch's The Reformation: A History. The students loved this book as a read but found it less helpful as a reference textbook and so I switched back to Lindberg the following year. MacCulloch is one of the best Reformation historians alive and this is what I would call a brilliant, scholarly beach read -- well-constructed explanatory narrative history, rooted in profound and accurate scholarship, laid out in the grand epic style. My guess is that Ref21 readers wanting a good, scholarly, readable history of the Reformation -- and one which will not break the bank -- should buy this.