People separate knowledge into two major categories. The first consists of areas where they believe some people know more than others (math, physics, computer programming, history, linguistics, etc.) and the second consists of areas where they don't (religion, politics, raising a family, etc.). The second area has always baffled me. Why will people willingly say they know nothing about math, but will rarely admit the same thing about politics or religion? Why will people accept advice on a subject, from someone that has studied it in depth, if that subject is programming, but not if it has to do with raising their children? Why, in some areas of knowledge, do we equate studying with mastery, and in other areas we don't? I'll call this the "wisdom fallacy" – that people believe wisdom has no correlation with knowledge for some subjects, when really it does.
In my work experience, I think most people lump management into the second category. I have never heard anyone say "I am not a very good manager," or even "I don't think I would make a good manager." Yet they will regularly opine on management, and explain their management theory in detail to you. But, by definition, some managers have to be the worst, right? And haven't we all worked for managers that we think knew very little about management? What is going on? Why is management something that nearly everyone thinks they do well?
I think management is more like math. It is something to be studied. The only way to get better is to learn. Solving a math problem is all about picking the right tool from your tool box. You can't say "I like Laplace transforms" and use them to tackle any and every math problem, because they only work for certain situations. Management is the same way. All of these books about how to manage this or that are really just tools to be used when the situation is appropriate. Which tool you use depends on the situation.
Our brains have cognitive filters for some things. We tend to automatically ignore facts that contradict our religious or political views. It's not intentional, our brains just have a lot of information to process so they throw out contradicting information subconsciously. Don't let that happen in your role as a manager. Seek out contradictory information. If you think you are doing a good job, don't look for evidence to support it, look for evidence against it. If Michael Dell can let his subordinates evaluate him so that he can become a better boss, shouldn't the rest of us follow his lead? If you really want to be a good manager, then study management, get feedback, and realize that just like any other skill, mastery of it takes lots of hard work.