pastiche:

A novel written in the style of an author and usually borrowing an author’s characters, like Sherlock Holmes and John Watson. It has a long tradition in the Sherlockian world, from August Derleth’s Solar Pons to John Dickson Carr’s and Adrian Conan Doyle’s collaboration The Exploits of Sherlock Holmes to the current Death Cloud, which explores the adventures of a 14-year-old Holmes.