Intelligence can be abused and exercised, but unlike other stats, exercise will never bring it above its base value (or the value you have raised it to through gain ability).

Sources of abuse: mind flayer brain eating, zombie attacks. Sources of exercise: reading spellbooks, writing spellbooks, writing scrolls, learning a new spell, getting a major Oracle consultation for the first time, possibly some of the uncommon, non-farmable ways stolen from the things that exercise Wisdom.