Studying Tomes gives the Skill a certain amount of points. There is a scale under each type of Skill. When you get a sufficient amount of Skill points and the scale gets full, the level of Repair Skill raises. With each level, Weapon and Armour repair will take less hammer strength.
N.B. Tomes are divided into several grades. Tomes with higher value grant more Skill Points. e.g. Tome V gives more Skill Points than Tome IV.