John, I'm not sure that books really fall into such neat categories - or that they should do so. Think of Rifles, which is written in a way that appeals to a very wide audience (and has been extremely successful), yet contains a great deal of new, original material that will surprise and reward even the most expert of readers. What is its target audience? If I was trying to interest a 16 year old in our period, I would recommend it heartily and with enthusiasm - but so I do to people who have been avidly studying the subject for more than 16 years! Clearly in a book like this the citations must be discreet, but equally clearly it is essential that they are there.
This may be an extreme example, but lots of books cross categories, and even the writers of the most specialized works (except Martinien's Tables) should try to make them readable. Think how the annotations enrich John Hall's Biographical Dictionary of British Officers - how dry it would be without them.
Nonetheless, I think we are in general agreement.