Leave it.

Deciding that it wasn't your place to take someone's diary, you put it back on the ground where you had picked it up and continue your walk. Either someone else will find it and be given the same choice that you had or the owner of it will come back for it once they realize its missing.

That's what just happened, by the time you end your route and turn around to walk back home, the book was gone. With a shrug you continue on...but the thought of what was in that diary was starting to nag at you. What if it was a gold mine of fantasies that some teenager had written about? What if it was full of dirty pictures? Or maybe it was just a boring day to day life of some angst ridden teen...

I guess you'll never know, however, the question left lingering in your mind.