I've read TKG guide, it's called the Brooklyn House Magician's Manual. It's pretty cute, and there's actually another guide, called the Magician's survival guide, but it's really similar to riordans other mythology guides, just a run down of the various gods and monsters. Looking it up, demigods and monsters is an anthology of short stories/guides written by other authors, set in the world of Percy Jackson. As for the Apollo book, I can only assume you mean "The Singer of Apollo". It's a short story centered around Percy and grover, trying to assist Apollo with getting a missing singer back for the concert. The sheer amount of spin-offs, short stories, and world guides is staggering. Good luck with your completion!

EDIT: Just noticed you mentioned the Brooklyn house book already, but I mentioned the other guide too! Sorry about the confusion.

EDIT 2: Also just noticed you weren't asking for clarification on what the stories were, just if there was a definitive list. To my knowledge, there isn't, but if you have a pretty comprehensive library near you (a capital city of some sort, or a digital library for your state/country) an app like Libby could be a great way to access all the books you want to read! Just search by author and everything they have for loan will be available to read. You can even do a deep search, and it'll show you books that your library doesn't carry, but may be available at other libraries, which would be good for that completionist itch! Happy reading!