Recent book - Strangers find/leave messages in a library, notes in book margins, hardback comes with paper clues

[Alexandra]

A while ago I noticed some discussion about a book and now I can't remember the title.

The book was about a couple strangers who found messages and clues left in a library, left messages for each other in book margins. And the book itself had these messages in the margins and/or the hardback version came with separate paper clues scattered in the pages.

The book is a mystery (and possibly also fantasy), within a mystery.

Adult, or maybe Young Adult.

This is a recent book. Sound familiar anyone?

[Alexandra]

Nevermind, found it!

[Ingo]

Although this is solved, this reminds me of a book I have yet to read, somewhere in my many stacks of unread books:

House of Leaves
His sister made a record, "Haunted" which I highly recommend.
Wikipedia:
House of Leaves was accompanied by a companion piece (or vice versa), a full length album called Haunted recorded by Danielewski's sister, Anne Danielewski, known professionally as Poe.

[Ingo]

And one more for my TBR, thanks!