The one that got me interested in the science was A Field Guide to Dinosaurs by David Lambert. It's dated by now, but at the time it really impressed upon me just how many types of (non-avialan) dinosaur there were, beyond the half-dozen familiar ones that everyone knows.
Wait. 1989? I _was_ 7 years old at that time. So probably 1990 or even 1991.
(among other things, _Troodon_ was given as an ornithopod!)
(Incidentally, on a dinosaur trivia note... this was the book in which Lambert inadvertently erected the name _Coloradisaurus_ for the preoccupied prosauropod genus _Coloradia_. Apparently Lambert was aware that Bonaparte had come up with the replacement name _Coloradisaurus_, but he was unaware that Bonaparte had yet to publish the name. Thus the new name _Coloradisaurus_ came to be attributed to Lambert, not Bonaparte. AFAIK, _Coloradisaurus_ is the first and only valid dinosaur name to be coined in a popular dinosaur book.)