Original sentence: GURPS Discworld (Steve Jackson Games, 1998) and GURPS Discworld Also (Steve Jackson Games, 2001) are role-playing source books which were written by Terry Pratchett and Phil Masters which also offer insights into the workings of the Discworld and the power of narrative
Edited sentence: GURPS Discworld (Steve Jackson Games, 1998) and GURPS Discworld Also (Steve Jackson Games, 2001) were written by Terry Pratchett and Phil Masters and are not [only] wonderful role-playing source books, but also offer numerous insights into the workings of the Discworld and the power of narrative