Fixed the missing hint: a 4th-job skill that needs its Mastery Book (for example Chain Lightning at 10) now shows a greyed-out + button once you reach the level the book unlocks, instead of a live button that does nothing. Still to look at: the Chain Lightning 30 book not working on a level 20 skill, and the job-skill quests (Gritto, Maple Hero) - leaving this open for those.

Found it: the Chain Lightning 30 book (and 77 other Mastery Books — most of the '30' books plus several 20s) had no data behind them, so the game called them unusable. Every Mastery Book now reads its skills straight from the item, so CL30 works on a level-20+ Chain Lightning (50% success, as in the original). And the 4th-job skill quests are back: skills that come from a quest (Ninja Storm, Genesis, Meteor Shower, Power Stance, Concentrate, Snipe, the Beholder auras, Bahamut and the rest) now unlock through their quest, which sets their master level; after that, Mastery Books raise it as usual. New: the Bowman's 'Junior Balrog in a Different World' at the Insignificant Being. Skills you get at the 4th job advancement still start at master level 10. Nobody loses anything: any skill you've already trained keeps its level, and that level becomes its cap until you do the quest. Still to come: the quests for Ninja Ambush, Guardian, Phoenix, Frostprey and Battleship, which need their training grounds first — until then those skills stay at master level 10.