Dean Koontz-Odd Thomas, when I finished it, I was a bit numb at the ending, I had fallen in love with the characters.
The Shack is the only book that had me so engrossed that I finished it in a single sitting, up till 4 something in the morning reading it.
Halfway through the Lovely Bones I decided it had become garbage but after starting so good I had to finish the chore of reading it hoping it would find the thing it had lost somewhere along the way.
Stephen King- The Long Walk, or even The Running Man, or The Rage, and Roadwork. Of his books, the Bachman books seem to have the most relevance to today's society.
And of course the Dark Tower, before it turned to crap somewhere around the sixth book.