Depending upon who you believe, the best-selling book of the 90s was either The Pelican Brief (11,232,480) or "Men Are From Mars, Women Are From Venus" (circa 10,000,000)
I think it depends on the meaning of your question....books written in the 90s only? or any type of book? either way i think you have the answer already in the bible (great work of fiction) or the others listed.
Woodpam - not during the '90's. Chamber of Secrets didn't hit the shelves until 1999 (paperback) and the entire series didn't really become that popular until the paperback Prisoner of Azkaban was released in 2000. The entire series will probably dominate the bestsellers lists for the 00's.
I very much doubt The Pelican Brief sold over 11 million in the UK alone. That would be about one copy for every five literate people. Same goes for Men Are From Mars...