This is a good question. I did learn to draw very realistically using a tonal approach to figure drawing. It's funny because when I go to a figure drawing session, I still draw that way which shocks a lot of people who know my simplified style in illustration. I also love drawing environments and made that a big focus of my education. My first jobs out of school was doing environments for the entertainment industry and also for architecture firms. In doing that I had to draw very believable scenes using accurate measurements (for example, a table of specific length would have to look the way it really would in a room setting, etc.).
If you know you are going to be doing books, I would say that you don't need to learn to draw realistically per se, but you do need to know how to construct well. That means understanding how forms turn in space, etc. I use a LOT of construction information to inform how I stylize. In other words, my shapes aren't accidents, but choices. And that is the main thing I want to stress. Whatever style you land on, make sure it's a choice and not because you can't draw anything else.
Lastly, I am coming to believe that design is equal too or more important than drawing well. Having a pleasing group of shapes and lights/darks can look great, even in a primitive style. While drawing "realistically" but with no design rarely equals a pleasing image.