The Baker Street Letters, by Michael Robertson

Very clever and charming book! When lawyers and brothers Reggie and Nigel take over the lease to use a business space at 221B Baker Street, they unknowingly sign an agreement to handle mail that arrives for Sherlock Holmes. Nigel begins to get concerened about some letters, and the mystery leads them to Los Angeles to uncover a Chinatown kind of crime. Fun and really original and damned well done.