I think that would be a great idea.
I agree with those who think that a single creature with not as many variations and types as the dragon would spread such a book rather thin.
A way around that would be to use other, similar diverse groups, such as Giants, that have been suggested.
Another possibility is having books on whole "Monster Ecosystems" as it were - that are all related together - like a group of races one always finds in the same location, including the interactions between them, and so forth. Such interactions could vary from master/slave for certain evil humanoids to full partnerships or trading partners. This would be good for those races that do not have a rich variety like dragons or giants, but would give plenty of material for a good-sized book.
While even more general themed books, like on undead, might be useful, I'd like to see books focusing down a little more on the day-to-day details of individual races. Like the details you'd see on a field report of "Life Among the Orcs" to give a real flavor to what is otherwise just a few paragraphs in the Monster Manual.