After I have read a Spell Tome (Sparks, for instance), can I sell it, or do I need to keep the Tome in my inventory?

It gets destroyed the instant you read it.

The spell "Sparks" is then added to your magic section of the menu

The spell tome is destroyed If you gain extra copies of the spell tome, you can't relearn the spell (it says you already know this spell) so sell it to your hearts content.

sawadee krap

If you have one copy and use it, it is destroyed. If you come across another copy, you can sell it because it will tell you that you already know this spell. You won't get much gold for the additional copies if your speech skill is low, but if you are in dire need of gold, by all means, sell them.

^{Thanks for the comments. I guess that is what happened--I just picked up extra copies of the tomes.}