Yes, the CS and Player's Primer to the Outlands are pretty much the only sources that talk much about the Outlands. The CS pretty much sets up the Outlands as a place where you can set low level adventures for a party based in Sigil, and describes some of the more important sites. This is because of all the Outer Planes, the Outlands is the one that most resembles an ordinary D&D world, except for the diminishing magic as one approaches the Spire.
The Player's Primer is kind of gimmicky (this was when TSR was experimenting with products that included audio CDs), however, it expands a bit on the material given in the CS. The Primer tends to focus on the gate towns. If you don't know what they are, they're communites that that have permanent portals to the other Outer Planes; each town has a portal to one specific plane, and the gate town tends to reflect the nature of the plane it leads to. The Primer does mention other sites in the Outlands, but only gives them like a paragraph or two, where the gate towns all have at least one full page.