You sort of have to marinate yourself in the ideas of the book. But the big one is that you should have a very limited API. And each API function should do a lot of things. So a very narrow set of deep APIs make for the best programs. The other thing was indirection. Try avoiding it as much as possible. Languages which allow easy access to functions or lambdas or blocks being pass around, usually end up with 4 - 5 levels of function calls to get anything done. That makes things really complected together and makes it hard to reason about later.
and which book are you talking about?
Deep APIs were mentioned in "A Philosophy of Software Design" (John Ousterhout)