I've helped write a few books.
Gauge Fields, Knots and Gravity with the physics grad student Javier Muniain is the most popular, since it's an intro to lots of math and physics.
Introduction to Algebraic and Constructive Quantum Field Theory with my advisor Irving Segal and Zhengfang Zhou is the most hard-hitting.
Knots and Quantum Gravity is a collection of papers I edited
, based on a conference I ran. And
Towards Higher Categories, edited with Peter May, is a really nice collection of papers introducing people to various aspects of n-category theory. Two of these are free.
I want to write more books when I retire, and I'm already planning them out. I think I'll write them as collections of essays, which I'll put on my blog and the arXiv, and then assemble them into books. It's much,
much easier to write a collection of independent essays that an equal-length single-piece book, since getting everything to fit together neatly takes a lot of work.