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.

It can also be traded or obtained via the Auction House, albeit usually at considerable prices.