Really hard to answer just one :). Every book from Simon Sinek or John C. Maxwell are pure gold but there are so many other authors like Daniel Goleman, Jim Collins, Dale Carnegie, etc.
If I could only choose one book, I would say "How to win friends and influence people" by Dale Carnegie, mainly because it is a complete book that we can use for self leadership. It's my favourite book of all time and it teaches so much that I highly recommend it. Not only for leaders but for everyone.