@Xavi - I know a valley in Provence where ghosts of gladiators can be challenged for Mentem vis (GoF), forests around are governed by genius loci (GoF), can be searched for vis (LaL), young magi compete for high stakes at the Tribunal gathering (LaL)... basically, I took a lot from Tribunal books and removed geographic references. Seems to work 
Thinking about it, the thing I'd probably find most useful at this point would be a book of example covenants. A book that would have saved me a great deal of effort in 'filling out' the 'there-but-not-in-the-book' covenants of a given tribunal by picking some from such a collection and tweaking them to suit my saga would have been a welcome addition to my library - and still would be for other purposes.
Also fill it with plenty of adventure seeds and it'd fulfill decent niche I think
A kind of 'Magi of Hermes' but for Covenants.
I'd buy books detailing 2-3 covenants and their region, most definitely. However, I'd probably buy any Ars book: I'll even buy Rival Magic, which I will probably not use (the Augustan Brotherhood a possible exception). Whatever, Mr Nephew says about "scenario" material, it's something I'd like more and more.