That's a fantastic question. I have my reasons, which I will get to, but I want to remind you that this kind of question doesn't have a single correct answer. This is a classic "what do you think" question. If I were to ask you the question, I don't really care if you choose novel, poem, or textbook. I care about why you chose what you chose. I care about your reasons. On top of that, I do not even need to agree with you. I would be grading and judging based on the strength of your answer and your support. My recommendation to you is to pick the answer that you feel most passionately about, or pick the answer that you can most easily defend.
For me, a novel is the best choice (even if it is fictional). My major complaint with a textbook is that it is factual to a fault. You, the reader, are given information about what happened, but very rarely are you given a clear picture as to why something may or may not have happened. Your initial question asked about human motivation. Inherent in motivation is the why. Why was a person motivated to do something? The actual act is the factual information that a textbook would have. The logic and the emotions that led to the act are what a novel can describe. The novel, especially something like a historical fiction novel, can give you both the what and the why more clearly than a textbook.
There is a secondary question hidden in your initial question. "Would extensive research help in shaping your ideas?" Yes. More research gives you more data and evidence. It's why scientists run multiple tests. They want more data, so they have more evidence. More evidence allows them to make better and more detailed conclusions about what and why something happens. Extensive research into human motivation is going to give you more data. As your data and evidence increases, you will be able to see more commonalities and themes throughout history and cultures. All of that information can't help but give you a better understanding of human motivation.