I think the oldest books still in print are as follows:
NWT (1984 edition)
My Book Of Bible Stories (1978, revised with study questions)
All Scripture Is Inspired Of God And Beneficial (!990 Edition)
Life-How Did It Get Here? By Evolution? Or by Creation? (1985)
Reasoning From The Scriptures (1985, and revised) as noted already
Insight On The Scriptures (1988) as noted
Revelation-Its Grand Climax At Hand! (1988, revised 2006)
The Bible- God's Word Or Man's? (1989)
Mankind's Search For God (1990)
The Greatest Man Who Ever Lived (1991)
Kingdom Proclaimers (1993) as noted-probably will never be revised and updated
Is There A Creator Who Cares About You? (1998)
Pay Attention To Daniel's Prophecy (1999)
Am I missing any from before 2000?
(YPA vol. 1 came out in 1989)