UI design is essential to any startup, but it's often something that's a secondary skill for startup founders, who have either a business focus or a technology focus, and who tend to treat UX as just another task on the list (if it's even there at all).
Of course, UX makes and breaks startups, whether consumer-based or B2B, and should be a central discipline. This list of books is a good starting point to improve your skills in that area.