You do not find such a tome. They don't exist in BG2. But yeah, what is that about? Goblins definitely don't have an 18 str. There are other strength enhancing items to be found in the game.

What is it with composite longbows. My main character is an archer with 17 strength...so no composite longbows for him because you need 18 strength. Okay, fine...I thought. But then it seems every puny bandit, kobold, goblin and orc can use them (yes, I find them on their bodies). I can't believe they are all strength 18!!! I'm on the lookout for a strenghth enhancing item (don't tell me where to find one, please) but I'm feeling pretty hard done to.

You can find a tome of gainfull exercise .
(I think).(I found itt in BG1).