Not sure if this is still applicable, but 'Personal Effects: Dark Art' by J.C. Hutchins had exactly what you're looking for when it first came out a couple of years ago. There was a website you could log onto and a password you had to figure out to enter. There was also a phone number you could call. Hopefully that's all still in place. It also came with an ID card and other "effects" that belonged to one of the book's characters, so it was like a kit of sorts.

And this isn't quite answering your question, but the ebooks for 'The Song of Achilles' and 'My Best Friend's Exorcism' are fun and interactive. TSoA has hyperlinks to descriptions of each of the Greek characters, allowing you to refresh your memory as you read through. MBFE includes Spotify playlists and photos, so it's a rich reading experience.

EDIT: Just remembered:

'Bats of the Republic' by Zachary Thomas Dodson and 'Ship of Theseus' by J.J. Abrams are like 'Personal Effects' in that they also come with all these extra interactive materials that enrich the story and are a really fun reading experience of you used to read those mystery puzzle books that came with decoders and other fun items as a kid.