It will all make sense if you assume that the author's head has exploded. The debris the author refers to is the contents of their head - since it exploded, it would have been splattered all over the walls. The author is therefore saying that the book contains all their thoughts and beliefs and so forth. By saying that their head exploded, they're implying that they learned something that caused their brain to malfunction catastrophically. Obviously, this isn't true - it's a joke.

That sentence was meant to be a joke. What specifically it's referring to would depend on the context.