Hands down, Infernum is and seems fit to remain my favorite "demonic/diabolic" planar layout of all time. Even as a hardcore fan of the World Axis, D&D has never made its demons or its devils or any of its other myriad fiends as inherently interesting and gameable as this setting does. It was Mongoose Publishing's finest work. Being focused on the domain-running, social mechanics and mass combat angles, this is probably the least immediately useful of the Infernum trilogy; it's definitely worthwhile getting it to complete your collection if you can, and the social mechanics angles can really add a lot if reverse-ported to D&D 3e or Pathfinder, which traditionally have been rather lacking in those departments, but it's just not as inherently useful as the Book of the Damned and the Tormentor.