This isn't actually a book so much as an actor's script for a play. Sherlock's Veiled Secret is by K. C. Brown, and I found it for sale on Barnes & Noble. The premise is that Violet Sheridan, a sculptress in 1920, finds out that Sherlock Holmes is her father, and then searches for her mother's identity. He also involves her in solving a mystery with him. Watson is absent because he's supposedly on a honeymoon cruise with yet another wife. So no slash is to be found; it's hetero love interests for all the characters.
Violet looks in Watson's stories for candidates for her mother, and these are revealed as Mrs. Cyril Morton nee Violet Smith, Irene Adler, and Agatha the housemaid for Charles Augustus Milverton. (There's a mention of Violet Hunter too, but nothing comes of it.) Holmes had some romantic relationship with each of three women, but his daughter only learns this by asking around. He won't give her any info himself, and keeps trying to push her to his blackmail case. There are some flashback scenes in the play, but I have trouble picturing how they can be staged to effectively go back in time 20 years using the same actors. I found the solution rather convoluted and felt that the mother character didn't give a complete enough explanation of how she concealed her pregnancy and left England to leave the baby Violet at a school to be cared for with anonymous donations. But it's an interesting exploration of Holmes's character and how he dedicated himself so totally to his career that he would never marry despite his romances.
Here is a review of the play by someone who actually saw a performance. He also links to a scan of a newspaper review of the play. It's hard to read, but I was able to make out the fact that K. C. Brown is apparently a Canadian woman. I wonder if she's written any other Holmes plays or books. The copy I have says the play is based on an earlier story she wrote with someone else.