However, in defense of Boas, you need to look at the intent of her book that she laid out in the Preface (2nd Ed.):
What she intended to do was to make sure that you did not see the phrase "orthonormal" or "eigenvalues" for the first time in a physics class. For many of us, we had to learn the mathematics at the same time as the physics. This is something she's trying to avoid. But this means that she has to deal with students who are still early in their undergraduate years with limited mathematics sophistication. That is why the book has to be, in many instances, superficial in some of the depth of the material being covered, and as she has said, covered only special cases. Many of these special cases are what physics and engineering students would have seen or will see, such as heat conduction problems and Gauss's law. It is also why this text is less "advanced" than say, Arfken text. But for the target audience and target purpose of what she's trying to accomplish, I don't know of any better text than this.