It wrecked me when I read it the first time, and it was the first time I experienced a writer ripping apart my brain and leaving me to jigsaw the thing back together.
A lot of his books could have done it, but that was the one that did.
My son caught me off guard asking me this question the other day. My first reply was The Stand, but there's a little book called Lying by Sam Harris that was pretty important to me.
Anyway, the reason I really replied was to say that many of John Grisham's books are kind of like candy, they just go down so quick. Fun reads by and large; The Partner is probably my favorite.