If I remember right, Tomas rarely appears after the first trilogy and only ever gets in the middle of the action in these books:
- Magician
- Silverthorn
- A Darkness at Sethanon
- Flight of the Nighthawks
Tomas may have a minor impact on the story in some other books, but those are as I recall the only books where he actually gets something to fight. In Magician and A Darkness at Sethanon he plays a big part of the story as well as the action. In Silverthorn it's a pretty unimportant fight. In Flight of the Nighthawks, he fights one person and barely wins, and doesn't kill him.