There are many good classic motivational books. One is The New Psychology of Success by Carol Dweck, a book where the psychologist Carol Dweck explains the concept of a growth mindset and how it can help a person overcome obstacles.
Another one is Man’s Search for Meaning by Victor Frankl, a holocaust survivor who talks about how one thing that helped him get through the holocaust was a sense of meaning and purpose and that those who have meaning and purpose are able to endure much more than those who do not.
Another good motivational book is The Obstacle is the Way by Ryan Holiday which introduces the ancient Hellenistic philosophy of Stoicism which teaches the importance of accepting difficult circumstances but still striving to do the best you can in the midst of them. Another one is Drive, by Daniel H. Pink, which talks about what Pink believes truly motivates humans including self-actualization, need for autonomy, and service to others.