We first meet Ponder in Moving Pictures where he passes his exam with flying colours due to him getting the wrong paper. I think we next meet him in Lords and Ladies, but I'm not certain. By that stage he's become the Reader in Invisible Writings. He was pretty unambitious to start off with and was getting ready for a nice quiet life in the UU eating large meals and sleeping a lot.
In L&L he goes along with Ridcully, The Bursar and the Librarian to Lancre to attend the Royal Wedding. It's kind of suggested at the end that he was going to stay in Lancre to study the 'thunderbolt iron' in the Dancers standing stones. There's a slight suggestion that he may have had a romantic interest in Lucy (Diamanda) Tockley.
However, by Soul Music he is very much back and more like the Ponder we know now. The bit with his ear is in (I think) Interesting Times, when he is shot by a lead pellet fired from the trunk of a ceramic elephant.
He plays a large role in The Last Continent, where he is tempted to leave the university and become the assistant to a Creator.
He's one of my favourite characters.