Literally five minutes ago (after more than 100 hours of gameplay) I randomly encountered the NPC that the journal is relevant. It wasn't even clear at first, I just started talking to him and his story sounded unusual. I asked him for a few more details, and then the quest clicked. The book was for him.
The amazing thing was that I honestly wasn't expecting it. I had no idea who he was, or where he was. I didn't really even have his name. All I knew was his story, a story which I randomly chanced upon the other side of as I was doing mundane chores. I hadn't googled the answer, I hadn't looked up the quest on UESP, and I hadn't even looked at the journal entry for the quest mark. I will tell you one thing, though, that was the most awesome quest I have ever done.
Not because the reward was expensive. Not because the story was epic or that the setting was amazing or anything like that. It was the most boring quest you could get: bring book to NPC. What made it incredible was the way it happened. I read this very compelling story in a book, kept it close to me for a long time, and chanced upon its conclusion. It was a pretty awesome feeling, and I want to thank Bethesda for that, and this game.
However, I would like to point out one little thing now that I've said that. This quest never would have been possible if I had just followed the magic pizza slice. This quest would have been boring beyond belief if a magic arrow just popped up and told me where to find this "long lost" person. How dull would that have been? So anyway, I would like to encourage Bethesda to take this into consideration when creating their compass, markers, maps, and journal entries.
Some quests make sense having map markers. If an NPC says "hey, go to my house, here, and get this thing" then yes, obviously they would just show you on a map. But for these quests about finding people, or things, that have been lost for years or centuries... Bethesda, I beseech you, it ruins the magic when the compass just says OH HERE IT IS KTHX FOR PLAY GAME.