From the publisher's website:
Terrain Toolbox is the first in a planned series of books that offer advice and tools for DMs to create more memorable encounters. Terrain Toolbox is designed to increase creative use of terrain in your game, making battles more dynamic.
The book begins with general advice for designing terrain effect and powers. It discusses why we use terrain, different things terrain can do, and how the terrain might change throughout an encounter.
Terrain Toolbox also contains forty-eight sample terrains, including blood mage’s circle, solid clouds, and lightning pillars. Each sample terrain includes advice on how to use the terrains, and most include mechanical and cosmetic variations.