I'll tell you why I'd prefer a book rather than an AP.
AP modules tend to include EVERYTHING that is officially approved and canon. In a hypotetical Numerian AP you'd be likely to find races from the Advanced Races Guide, monsters from the Bestiary 3, classes from Ultimate Combat, archetypes from the Advanced Players' Guide, spells from Ultimate Magic and so on.
If, in your campaign, you only use Core Rulebook material, with just something from the APG thrown in to add flavour, you'd be in for a lot of money to spend or a lot of work to do to convert/adapt/adjust encounters and NPCs.
Give a me a book on Numeria with some basic information and let me build "my own" Numeria based on what I use in my campaign. I still may need to make adjuments and corrections for the occasional Dhampir ninja/magus NPC, but it should amount to less work than having to rewrite half of the adventure.
OK, now, having said that, I still would totally dig a Numerian AP. :)