My personal favorite author, Herman Wouk, did this brilliantly. In Winds of War, the author created a character, who, then wrote a book reviewing the military tactics of the Nazi's. In the novel, the MC finds and reviews this book which gives the Nazi perspective. The MC then makes notations and corrections based on the facts alone. Although dry and factual, the Nazi officers book gives a view into their war machine mentality.

It all blows my mind. First off, the novel itself is incredible, then to create a character, and THEN write another book within the novel. Mind blowing and done to perfection.