I discovered Koontz around 1994 through one of my parents, though I do not recall which. My mother enjoyed his penchant for writing great dog characters into his books, while my father had copies of Stephen King works, so it's a toss-up into who was more responsible for the novels entering my household. Regardless, when it came time in middle school to start devouring 'real' novels, Koontz was at the forefront.
There is not much I recall about the books in general. I remember that Watchers was likely his best, Phantoms was likely his scariest (the first half is remains quite effective, before Koontz's trademark government conspiracies and psychopath killers get shoved in), and Twilight Eyes was likely the first sex scene I ever read in a book - this last one is quite ridiculous to return to, and probably gave me some wrong ideas about human intimacy.
Still, while I had been in love with the horror genre beforehand, Koontz provided a gateway into the wondrous addition of 'suspense thriller.' But looking back, what struck me most about his works were the covers. From the late 80s until the 1992 release of Cold Fire, Putnam re-released a wide array of his writing in new paperback editions that remain some of my favorite cover artwork. Simple yet incredibly evocative and creepy, I wonder now if the images played a role in my love of motels and carnivals.
I spent time around 1994 to 1996 deep into these works, and while I continued thereafter with Strange Highways and Ticktock, the last I read was 1998's Fear Nothing. Perhaps I had grown tired of Koontz's oft-repeated tropes, or perhaps I thought that reading Koontz was no longer 'cool.'
But it was probably because the covers just stopped being awesome.