hello bob, that's great news about your book, congratulations ! and making it available as a free download before it's published is a brilliant way to get some publicity and start gaining readers. i believe cory doctorow (among others) managed to make a name for himself partially through free ebooks which allowed new readers to discover his work.
hopefully it will continue to be available as an ebook after the paper version is published as well ! if harper collins has no plans to do that, and you've retained the digital rights, you might think about publishing it yourself via any number of the distribution plateforms available for writers. or if you want to continue giving it away, take a look at
feedbooks for an excellent publishing and distribution plateform. in addition, there are plenty of people here on MR who would probably be happy to help you with the format conversion in exchange for a free copy of the book.
please do let us know when it's available ; i love mysteries and i'm always happy to find a new author. cheers !