Shape is immensely important in Go, and you can read about the basics anywhere - don't make empty triangles, table and mouth shapes are good, etc. Where can I find more advanced treatments of shape?
There are insanely many tsumego in existence, but many of them are very artificial and would never occur in real games. While any tsumego helps to improve, specifically learning shapes likely to be ...
I recently took A. Dinershtein's Go style test. It was useful to me, as the test is aimed at finding professional players who might think the way you do, for study purposes. It has definitely worked ...
I think I can learn more if I see what people some kyu above me do. I'm aware of a related question, but I think, this one is more specific and might diverge.