Mr. Pressius appeared in the second and the third books. In the second (The Mysterious Benedict Society and the Perilous Journey), he was a "bullfrog" on the Shortcut which was a fast boat that the Mysterious Benedict Society traveled on. Mr. Pressius was showing off his precious diamonds where one was fake and the others were real which was a part of his plan so he could fool people who might've wanted to steal it with the fake. When the boat lurched, the diamonds all fell and the fake and the real diamonds got mixed together. Reynie helped solve which were real or fake by saying that if they were all put in a tub of water, the real would sink to the bottom and the fake would float.
In the third Mysterious Benedict Society book, he came to Mr. Benedict's house with forged papers declaring that he was Constance Contraire's father. Constance then uses the Whisperer to get her memories back and learns that Mr. Pressius is not her father so Mr. Pressius (who is thought to be working with/for Mr. Curtain) couldn't take Constance.