I haven't watched Goonies in a while, but I would bet on a book. Possibly a book on the history of Astoria, as a reference for his quest. It would describe One Eyed Willie's role in history. Therefore, this book would not exist in our real life history of Astoria. The baseball card would be a bookmark, marking an important page for reference. There would be little need to mark a page in a Journal, since if he would have created the journal, he would know where most things are or just remember the information without referring to a journal. You can see if there's a history of Astoria that's about the same size and thickness. Though, I kind of doubt it.