Any book worth reading is worth reading slowly.
What's the slowest book you've read?
Math is slow because you have to work things out as you read it.
Books written very long ago are slow because I read them like a detective, looking not just at what the author means to say, but also at what he's saying implicitly about how things were at the time.
Other books I read slowly because they're so good I don't want them to end. I used to have to make a conscious effort to make Patrick O'Brian novels last, and I stopped reading them at about number 12, to save the rest for later. (I worry though that I wouldn't like them so much now.)