What happens if you use a Tome of Water on your pet? Is it worth using, or should I sell it?

The Tome of Water (as well as Tomes of Darkness, Death, Earth, Fire, Life, Light, and Magma) raises 1-3 points of level, 1-3 points of strength, 1-3 points of defense, 1-3 points of health.

As for whether or not it's worth using, that's up to you.

It would depend on your luck, if however, you wish to acquire more tomes, you have a 33% chance of getting one from mage amulets.

Oh, ok! Thank you both very much! ^_^