The Next Parish Mail: The Clues So Far

I'm hard at work on the next Parish Mail book, and this one's got a unique angle that I'm really excited about. All will be revealed on Monday, July 29, but here are some hints to get you guessing:

1. It has its own soundtrack.
2. It's got some special characteristics in terms of both format and content.
3. It's a concept I discussed very early on with my publishers at Coliloquy, before even coming on board as an author, and is pretty much unique to ebook publishing.
4. It's inspired (but not based on) one of the lesser-known Sherlock Holmes mysteries by Arthur Conan Doyle.
5. It's related to a "holiday" celebrated one day during the week of June 23-29. Quotation marks intentional.
6. I and Coliloquy often use shorthand in emails when discussing the Parish Mail books: Dead Letter Office was PM1 and Post Mortem was PM2. This book's shorthand, however is PMX.

And the FINAL HINT:

If I was doing this right, there would be 12 hints instead of 7.

Watch this space on Monday for the reveal of PMX, as well as answers to questions about these hints!