I make it a hobby to collect rare and mysterious book series so I'm intrigued. This series isn't ringing any bells, but I've got contacts in some of the old bookshop circles. I'll shoot a few messages and see if anything turns up.

Mandela effect? Idk, sounds like the kind of series that would get merged in my mind with all the other detective stuff from childhood. Are you sure it was a series and not just one book you're building up in your mind?

Penrose the Cat solved the mystery of the disappearing book series by not existing in the first place. Maybe you dreamed it all? lol

Pro tip: Check out the Wayback Machine, it archives web pages. Maybe your library's old catalog pages are on there and you can find proof of the book series. Shoot, if you find something, maybe hit up a publisher with the info for a reprint campaign??

Hey, I think I've got an idea what you're talking about. There's a slim chance it could be a series that's now out of print, which is why your library doesn't have a system record anymore. I’ll brush off my bibliophile detective hat and see if I can help you dig deeper. Any other details you can recall might be useful, like character names other than Penrose, specific puzzles, or what the covers looked like?

Never heard of it, but sounds fun. Hope someone finds it, got a niece who'd love that kind of stuff.

Sounds a bit like the Sir Cumference series? That had a bunch of math stuff in it but it's knights not detectives. Maybe mixed memories? I'd help look up the book series for you but my librarian friend isn't working today 😕

omg I totally remember something like that too! It had those really cool illustrations right? Can't remember the name for the life of me but now it's gonna bug me lol.