It depends on your interests and expertise. If you're passionate about history, a historical non-fiction book could be a great choice.
One option could be a biography of an inspiring figure, or a book on a scientific topic that you have in-depth understanding of. The key is to focus on something you're knowledgeable and passionate about to make the writing engaging.
Well, it could be a memoir sharing your personal experiences or a self-help book offering valuable advice based on your knowledge. It really varies based on what you have to offer and what readers might need.