Publishing Hawkmoon sourcebooks for areas travelled by Hawkmoon himself (extracting all informations from the books and write it down as a background book, adding more details for logically filling the gaps) is useful.
But publishing sourcebooks for areas not covered in the books, except mentioning their names is not useful at all.
If we could get the original author of that setting, Michael Moorcock, to give us more informations (if he has any in mind) that would be fantastik, but a book written entirely by Mongoose authors, based upon NOTHING is not a thing i would like to see.
Mongoose authors can only use the same sources as i do: make these things up!
Cheers
Osentalka