Wisdom

Wisdom is one of the six character abilities. It's very important for clerics and druids, but rangers and paladins also have high amount of wisdom as a minimal requirement. Rangers and paladins however don't benefit from bonus spells, as the amount of spell slots and which level of spells they can cast are limited by their class rules. Although having high wisdom is not as important for wizards, be careful of not having low wisdom, as this makes wizards vulnerable to spell failure, as spellcasting is their advantage in combat.

The bonus spells are cumulative, a cleric with 15 wisdom has two level 1 spellslots and one level 2spellslot as a bonus for example.

Permanent wisdom boosts
Tomes of Understanding, three in total in Baldur's Gate (game) and Baldur's Gate: Enhanced Edition.
 * One can be found in Durlag's Tower.
 * Another one can be found in the Underground passages below Candlekeep
 * One can be found during the quest to remove the poison from your body which was injected by Marek, but it's required in to complete the quest.
 * In Baldur's Gate II: Shadows of Amn, one point of wisdom is gained when offering the last Tear of Bhaal in Hell.
 * Machine of Lum the Mad in Baldur's Gate II: Throne of Bhaal