@Justin Xu,
A tangent is a mathematical term for when you are on a circular path, but keep going straight. In other words, you are going off the path in some set discussion.
In this context, he is making a statement on something that isn't really the focus of what he is talking about. So he mentions it, but keeps going along his track. If you decide to follow this new direction you will be going off on your own (since the book follows a different path).