The question is: if a person reads a number of books about decision making or go to courses on this topic, does he need critical thinking too? The short answer: it depends.
One of the reasons to learn critical thinking is to become a better decision maker. A number of skills, methods, techniques and dispositions are used to improve different aspects of decision making. So you can’t learn an ultimate method and become the best. You start learning different skills that gradually lead to the final result when you learn critical thinking.
If you start with a decision making course you probably learn about different methods and obstacles to use these methods correctly. Also this course would focus on the same or similar aspects to ones you can find in critical thinking, because the process of decision making have the same pitfalls no matter how do you start practising it. As the result you will learn some methods and skills from critical thinking.
Starting learning critical thinking you most likely learn more different skills, because critical thinking is focused not only on decision making. Of course most of things you learn will lean to better decision making, but some of them are designed to produce some good intermediate result.
For example, you need good sources of information to make good decisions. Information often comes in a form of text, so many critical thinking courses teach how to check and analyse different texts. This could be useful if you work with texts often, for example as a teacher or as a journalist. In the same time decision making course can teach extracting correct information from texts and checking their origins, but I expect it having less emphasis on working with texts.
In the end decision making course will teach you some critical thinking so it is up to you how to learn. If you are a corporate manager and you want to improve your decision making fast you can start with an appropriate course. Later you can expand you skills with a separate critical thinking course, most likely your decision making will be improved too. And if you are more interested in critical thinking or if you want to study skills like text analysis, or focus on logic behind decision making, you can start with critical thinking course.
If you can’t make a decision right now, just look for a couple of courses on both topics and compare them. Or compare books. You will know what kinds of skills and methods each way proposes.