Tome of Polymorph: Turtle

Tome of Polymorph: Turtle teaches a mage the new ability: Polymorph: Turtle. Learning the spell consumes the tome. This spell is mechanically the same as Polymorph, except the target is transformed into a small turtle instead of a sheep.

Source
This tome used to drop from Gahz'ranka, a summoned boss in Zul'Gurub, a 20-man raid instance in Stranglethorn Vale. Since patch 4.0.3a it can be obtained from fishing in high Cataclysm fishing pools.