spin-offs

In recent years, more and more people seem to be writing all sorts of nonfiction books based on popular literary movements, TV shows, and other popular media. One of the most well-known versions of this seem to be theme cookbooks, including a Downton Abbey cookbook and even a Lovecraftian horror cookbook called "the Necronomnomnom," which came out just this year. However, other kinds of books exist, including self-help books with popular characters and even a guide to starting a business using tips from Game of Thrones. Are these tie-ins a clever idea or a cynical cash grab? Can they offer any insights that a standard book couldn't? Are there any examples that stand out as particularly interesting or useful?