A general holds 3 chosen skill-book slots. Add success rate: 100% for the 1st skill, 50% for the 2nd, 25% for the 3rd - and a FAILED add overwrites one existing skill. A higher-level book overwrites the same skill at 100%; you CANNOT add a lower-level book of a skill already present (this also blocks a golden general's native skill, e.g. Baibars' double-drop blocks the Luck book). Reliable method: first fill all 3 slots with Lv1 versions of your final skills, THEN overwrite each with the Lv4 book -> guarantees 3 desired Lv4 skills. Decide the final 3 before starting; keep >=3 Lv1 books of each (filling 3 slots took ~9 books). Extra books cost 3,000 gems each at the Academy skill-book shop. Exceptions: some generals accept only Lv4 books (Khalid = Siege Machine Attack, Barbarossa = March Size), so the Lv1-first trick fails for them.
Original research source: Evony Guide Wiki
