While I haven't actually gotten the Doomsword book, I can vouch that forging books do come from taking a stroll in the forest. You receive two books- mine were for the Angel Talon and some priestly Watcher staff. I got them while trying to get Arcane Edge... so it doesn't have to be your Blacksmith taking all of those strolls. Maybe one of your other Sims will have better luck? And, of course, having a super-high mood score will do wonders. I got mine when my wizard drank the fortune boosting potion... I don't think it actually did anything beyond provide a sizable mood buff, but I like to pretend it helped. Hooray for harmless superstitions.
...also, where do the hilt and blade come from? I haven't found either of them in the forest, village, or docks, so I'm kinda curious. Do you have to jump into the beast's pit or something?