From Discworld & Terry Pratchett Wiki
Young Caslong is to be found in The Truth, where he helps Gunilla Goodmountain and Boddony (all dwarfs) with the Movable Type Printing Device which enters William de Worde's life rather abruptly and is the catalyst for the events that follow.
The name Caslong is yet another sly nod to Roundworld. Caslon is a well-known typeface named after its creator William Caslon, who released it in the 1730s. It was a highly successful and popular typeface throughout Europe and America: the first printings of the American Declaration of Independence and Constitution were set in Caslon.