Not OP. I don't have anything against Strang, I think his teaching is very good, but it's worth noting that his approach to the subject is very applied and not

Not OP. I don't have anything against Strang, I think his teaching is very good, but it's worth noting that his approach to the subject is very applied and not very general. This is totally fine if applications are what you're interested in but for somebody who wants to learn more about the theory and the underlying concepts, there are better resources.