ACROSS THUNDER RIVER is a guide to the Westermarck and life along the Thunder River. It's also a guide to Picts and Pictland, with guidelines for creating Hyborians or Picts. There is new magic, new prestige classes, feats, lots of NPC writeups, and a gazeteer of the area. Plus more stuff.
THE FREE COMPANIES describes military culture in the Hyborian Age, with mass combat rules, detailed descriptions of all of the armies of the different countries, and descriptions of military societies and organizations. Lots of NPC stats.
There's a lot more to either book, but I really can't say more other than the obvious (NDA and all). Most of this information could be gleaned from other sources, such as previous postings on this very message board.
It would be very cool if the talented Misters Darlage and Kalvar could pipe up and speak more on these fine sourcebooks.