I generally read maths books by osmosis - if I read a theorem/definition enough times over some long period of time, then eventually I'll be comfortable with it. I always borrow several different books on the same topic so that I can look through all of them in order to find the approach that makes the most sense to me. If I really have to learn a topic then I might sit down with one book and read for as long as I can until I get bored/my mind wanders, which takes at most half an hour... And I definitely never bother doing exercises or anything, though if it's some new technique/concept I've learned, I might look through the exercises section just to check that I've learnt enough to at least understand the questions.
I do try to learn lots from random areas though - I check various blogs daily, as well as reading math.nt on the arxiv, and reading (and occasionally responding) on mathoverflow.