With normal current-day books that I can read off the bat without any problems, I don't really do that. I just read them from beginning to end as fast as possible, as time permits.
However, when I read something such as the Ancient Classics, or medieval literature, I read slower. My knowledge of history and languages is extensive enough that I can understand that kind of literature without having to look up every other word or location, but sometimes I do have to think about what a sentence means, or where a city was exactly located, for example. If I really don't know or can't remember, I look it up.
I've finally discovered a purpose for the Kindle Browser
I wish I had a 10.1 inch 16:10 e-ink tablet, so I could read two pages side by side, but also have more space to navigate websites such as wikipedia.
Strangely enough, reading modern-day books feels like "consuming" a story, while reading the (ancient) classics actually feels much more like reading. Maybe it's because it's a bit more difficult or something, or because it's language from another time; I don't know.