Depends on the book. I kind of like to wallow in a big chunky epic fantasy. The middle is the best place to be. For fantasy and SF there is a cost to starting a new story as you have to adjust your mental gearing. But for classic crime, they're almost all under 200 pages, and that feels right, too. They cover much shorter time frames, there's no world-building, and the encompassing series tend to go on for ever.
If I'm reading a book that is both big and slow, though, I do start to think about what else I could be reading.