Is the solar system stable? What stops planets from just flying off into the sun or into deep space?
It's at first natural to say "conservation of energy + angular momentum", but this actually isn't quite true. You could imagine that over time, inter-planet interactions would distort orbits to the point where the solar system gets disrupted. Certainly, asteroids, comets, and moons sometimes collide or get ejected from the solar system. And dynamical systems in general can be chaotic, and can transition in surprising ways from stability to chaos. The real answer is more complicated and this book tries to explain it.
The first part was a rehash of Ptolemy, Copernicus, and Kepler that didn't add anything to my understanding. The interesting bits start about a third of the way through when the author starts talking about Newton, Laplace, and classical efforts to prove that the solar system was stable. The author then highlights the dynamical discoveries of Poincare. The final third discusses late-20th-century computer simulations.
The real answer about solar system dynamics seems to be "the solar system is chaotic, but only within limits." The basic orbital parameters, such as planetary orbit diameters, seem fixed. But some of the other parameters do vary chaotically. There are funny resonances that result in some dynamical variables, such as the eccentricity of Pluto's orbit or the rotational axis of Mars-- changing chaotically.
As a student, I took courses in dynamical systems, and I've worked on simulating them as a researcher. I felt like I would have had a hard time understanding this part of the book without that background. I get the concept of a phase space in a fairly intuitive way, and so it makes sense to me that a system can be chaotic in some dimensions but not others. Readers who don't have the same background as me might find this all a bit heavy going.