don't forget that these maps are actually inaccurate, especially for sigil, since it's ever-changing. not quite as much as limbo, but having played through torment you should be aware of this.
now, for the planescape sources.
THE source is
www.planewalker.com with its excellent planescape campaign setting books (free of charge, pdf, they have some sort of agreement with wotc so they can use official content to some degree).
you may also want to visit their forums, some projects in there are quite interesting, too, like urban planescape (pscs mixed with d&d modern).
another good one is the mimir:
Planescape: Welcome to the Mimir
previous official planescape fansite, now on hiatus. it has a neat dictionary of planar cant, i think it contains over a hundred terms.
here's some info on ordial plane, the stuff is hard to find:
The Nature of the Elusive Ordial Plane
and an introduction to genasi, including the quasielemental, which are quite rare, too:
The Roaming Genasi Tavern
aand a good place to look for more is the planescape open directory:
Open Directory - Games: Roleplaying: Genres: Fantasy: Dungeons & Dragons: Campaign Settings: Planescape
also, the novels aren't bad. and there are official books, the stuff from 2ed is more comprehensive afaik, except for stats (esp. for deities and good/evil exemplars).
also, google helps, as always.