Message in a book

I remember browsing a used bookstore (they're the only kind to visit) and, from time to time, finding a note lying between the pages. Sometimes they were cryptic notes like "Meet me by the clock tower" and sometimes they were things like receipts or movie tickets used as makeshift bookmarks. Although each gave a little window into the life of the previous owner of the book, or just someone who picked up the book to leave the note, the cryptic ones were the most intriguing. They were left on purpose to be found and read by someone, but by whom and when was unknown.

And now I'm thinking of doing the same. I'll do it in English and perhaps it will add more mystery to the note.