by Robert A. Heinlein, Spider Robinson
The best Heinlein book Heinlein never wrote! That is to say, Spider Robinson's novel, based on notes and an outline never completed by Heinlein, is a credible entry into the universe of all things RAH.
Robinson happily lifts concepts and notions (slidewalks, Nehemiah Scudder, Harriman industries) out of the Heinlein canon, and merges them with his own ideas to create a story that fits well within the Heinlein Universe. Dramatic, and picking up on ideas and technologies that Heinlein never saw, this book does feel like it fits in with RAH's own work.