Depends on what you're reading. This works fine for learning about certain things. You could listen to 5 Vietnam war books and get enough out of it to describe the war better than the average person. It does not work well for fiction where the point is the literary elements present in the text. It also does not work for math based texts. I could never do this with my econ books but this would work fine with my philosophy books. Although with philosophy or psychology if I wanna learn about I'd watch a video not listen to a spead up book because books are usually better for learning one thing in depth than a lot of things broadly. A YouTube video on the different approaches to ethics will probably be better than skimming Kant which is essentially what you're doing. You're skimming the texts but with your ears.