Right now I'm reading "The Black Swan." Next, I plan to read "Surfaces and Essences." So I would say popular science recently.
I know textbooks are designed to do the work of drawing out main ideas, explicitly define main ideas, etc., but I would still be interested in learning how people approach textbooks.
I've already read about how to read scientific papers, which seemed like a great resource: http://violentmetaphors.com/2013/08/25/how-to-read-and-understand-a-scientific-paper-2/
Popular science books are often heavily padded. I recommend skipping any sections that are just rehashing the main idea. For some books (maybe even most books) this can mean reading the first few chapters and skipping the rest.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.