There are a few reasons why I'm not struck on a Book of Cults, despite its obvious utility.
1. It'd be an utter bugger to write. What do you include? Some pantheons could fill a book on their own. What do you leave out? I have, for example, J A Coleman's 'Dictionary of Mythology' which is one of my key references sources. Its by no means exhaustive but covers, in one to two sentence descriptions, just about every culture, god, hero and myth in the real world. It runs to 1200 pages with a small font.
2. Does the magic always fit the cults chosen? If not, do you use valuable book space to describe new magic systems to fit? How far do you go?
3. You'll always be sacrificing context, which is all-important, especially for myths. Its that context that makes the cults significant, which is why in Cults of Glorantha and Cults of the Young Kingdoms there's such a lot of weight given to it.
Its far better, IMO, to keep cults to their historical context where the appropriate depth and cultural relationships can be focused on properly. 'Deities and Demigods' was really nothing more than a list of AD&D stats for gods. Clearly RQII wouldn't handle them that way, but assembling a satisfactory - and satisfying - generic bulk cult book would require a huge amount of effort and a huge page count to do things properly - at least, to do things in the way we've established with RQII.
So I'm firmly resistant to such a book. Not to deprive you all, but because I know, first hand, how tough books like this are to write if they're to be done properly. We could churn-out a Cults Encyclopedia without much trouble, but just about every cult would be very trimmed down to the point that the all important flavour, mythical context and cultural context would be missing. If you'd like an example of this, take a look at the AH 'Gods of Glorantha' boxed set. Zillions of cults in there; none with any real meat or detail.