I have to say, I was a little surprised to see low reviews for this book. I understand what the reviewers are saying. They think nothing really happens in the story and it's slow.
I thought it was great. From a Buick 8 is more of a psychological horror story, and a tale about how there are some things we just can't know. I found every bit of the mystery fascinating. Some parts had me figuratively at the edge of my seat, or literally cringing with fear.
Many think this book ties in with The Dark Tower, and I agree. The titular Buick, not really a Buick at all but some sort of car monster, is the sort of car the low men from Hearts in Atlantis drive. I believe the black-clad driver from the beginning is one of the low men, or can-toi (rat-headed men from The Dark Tower). The cars are what they use to round up their "strays," and possibly what they use to transport them from any number of Earths to Mid-World. Saying any more would get too far into spoiler territory, so I'll leave it at that.
I will admit that this was a bit of a slower read for me than other King books, but I still found it a gripping story recommend it highly.