Heroes of Battle.
It taught me an awful lot about DMing. That wasn't its goal, I know, but it accomplished it. There were two specific things that I really found insightful. First, it discussed using diagrams to help get a handle on somewhat freeform outdoor combat scenarios. The system they suggested was really useful for planning any sort of pseudo-freeform plotline. Second, its advice on how to run massive battles was really great. Build on the diagram system. Have the PCs engage specific foes or scenarios within the context of the larger battlefield, and then have the battle outcome affected by how the PCs fare at their tasks.
Like a lot of DM advice books, the information inside was really obvious once I understood it, and I didn't need the book much once I'd absorbed its lessons. But that's ok, it was worthwhile reading.
It taught me an awful lot about DMing. That wasn't its goal, I know, but it accomplished it. There were two specific things that I really found insightful. First, it discussed using diagrams to help get a handle on somewhat freeform outdoor combat scenarios. The system they suggested was really useful for planning any sort of pseudo-freeform plotline. Second, its advice on how to run massive battles was really great. Build on the diagram system. Have the PCs engage specific foes or scenarios within the context of the larger battlefield, and then have the battle outcome affected by how the PCs fare at their tasks.
Like a lot of DM advice books, the information inside was really obvious once I understood it, and I didn't need the book much once I'd absorbed its lessons. But that's ok, it was worthwhile reading.