The sub title of the book is, "My war with Stephen Hawking to make the universe safe for Quantum Mechanics"...and that's pretty much what it is.
The book shows the 20 year chronicle of a battle of ideas between the Relativsts, or believers in the Theory of Relativity, and those who insisted that elements of their ideas were wrong. Specifically, the book posits the idea of 'information loss in Black Holes', which basically meant that Stephen Hawking in the late seventies posited the idea that once something goes past the event horizon of a black hole, it is 'lost' the universe. Susskind's problem with this is that basically it violated the second law of thermodynamics.
Over the next twenty years, Susskind outlines radical new theories in physics which pretty much prove him correct, including the Holographic Principal and String Theory. Some of the work is theoretical in nature, but some of it relies on rock solid mathematics until Stephen Hawking himself, no more than two years ago, conceeded that information loss in Black Holes was not likely.
The fascinating thing about all of this is that almost all of the work was done entirely by thought experiment. Just the way that Einstein came up with the Theory of Relativity in the first place. The book is an excellent way to become familiar with some of the more complicated subjects in Quantum Mechanics and Cosmology. It has very little math, and the narration follows a pretty smooth style.
I liked it and learned a great deal from it.