OK, the library is worth it.
I’m still playing around with Bannerheart, a mature succession fortress because nobody else has picked it up. “Skill up very quickly with the Library,” the manual says. It isn’t linear and hadn’t seemed to me to be worth the time compared to just making goods. “A legendary armorer is arguably the most valuable dwarf in the fortress” says vanilla Wiki and that’s perhaps true but for Masterwork mod, it seems to me a Weaponsmith is more valuable: good armor is good but better is not to be hit in the first place and in vanilla, the worst invaders are gobbos with iron. Masterwork in a mature fortress faces Frost Giants with skills, good weapons, warbeasts with names and steel armor so, taking them out as soon as possible is best.
Bannerheart’s best weaponsmith was level nine. Not too bad, made decent weapons. Reading ten smithing essays: level 14.26 => 14.34, not worth the time. Second attempt with ten essays: 14.34 => 14.48. But then there were books. Reading ten books on smithing, nine became masterpieces: 14.48 => 15.37. Ten more, 16.2 within a week.
That’s level 9 to Legendary within a week. A legendary weaponsmith not only makes better weapons but also very valuable trade goods (☼steel trap component☼ = 36K; mithril 68K, bifrost 151K). That’s a waste of bifrost but I was curious. In v.3h, the Drow came first winter with a dozen Books of Summonings; those would have been many legendary skills from the Colosseum if affordable.
Now we’re skilling up the best armorsmith; I still want good volcanic breastplates.