1989-1992, three years, 14 books and a short story:
The Unadulterated Cat (1989)
Pyramids (Gollancz, 1989 June)
Guards! Guards! (Gollancz, 1989 Aug)
Good Omens (1990 May 1)
Eric (Gollancz, 1990 Aug)
Moving Pictures (Gollancz, 1990) (probably Sept)
Diggers (1990)
Wings (1990)
Reaper Man (Gollancz, 1991 May)
Witches Abroad (Gollancz, 1991 Nov)
short story Troll Bridge (1991)
Small Gods (Gollancz, 1992 May)
Only You Can Save Mankind (1992 Sept)
Revised The Carpet People (1992)
Lords and Ladies (Gollancz, 1992)
Then he took a little break and only wrote two books and the first Mappe in 1993:
Johnny and the Dead (1993)
Men at Arms (Gollancz, 1993 Jan)
Streets of Ankh-Morpork Mappe (1993 Nov) - I seem to recall he said the short material - diaries, short stories, booklets for the Mappes - took as much work as a novel, in part because they had to be kept short.