A bit strange, usually if something falls through the map it teleports to the top of the world and falls near where it fell down- it might be in a bush somewhere nearby but it's probably easiest to put the password changer in the mailbox, save, start a new save, run to the mailbox, get the password, changer, and get the book in the new save and put the password changer and book in the mailbox and save. You can do the same with anything you can put in your inventory- hopefully the book can be picked up (don't see why it couldn't be) as that is the only requirement to using the mailbox besides being in story mode.