Other reviewers have gone into detail about what the book covers, so consider this more of a testimonial.
This is a book that any GM should have. Period. It has gotten me past GM's writer's block. It has helped me finish half-formed ideas for story arcs.
The fact that it's available in pdf makes it even better. While the indexes are impressive (to say the least), the ability to search for a keyword like "horror" or "space opera" and flip immediately to those entries is a huge value.