I've only rarely been a successful autodidact, not for lack of trying (I did teach myself etching many years ago). Several years ago I tried to learn motorcycle repair through an online course, and in fact received my certificate of completion, having read all the booklets and passed all the required online tests. I didn't get much better at fixing motorcycles, unfortunately; what was missing was the teacher.
Thorough knowledge of content is an absolute requirement for a teacher, but a textbook covers content thoroughly as well, and more systematically and comprehensively than a teacher is likely to be able to. So what are teachers for? The delivery-of-content model of instruction doesn't describe all of what teaching is about. Most of what you really must know and be able to do isn't delivered to you.
You don't need someone up there who knows all the answers to tell you what to do.
"Math is not about following directions, it's about making new directions," says mathematician and teacher Paul Lockhart. A good teacher may not in fact know your answers, but they have a role in getting you to your them.
Key to this is recognizing that learning is as much a matter of doing on the part on the student as it is explaining on the part of the teacher. This model applies to every subject, as educators are increasingly recognizing; arts teachers seem to have always known this, except for writers of how-to books.
What is the role of the teacher?
A how-to book, in removing the presence of the teacher from the teaching process, removes much of what makes learning personal, meaningful, and lasting.