If you're an endurance athlete of any kind, Dean Karnazes' book 50/50 is a "must read". It's the story of his amazing journey to run 50 marathons in 50 consecutive days.
The people he meets, the stories he shares and the shear magnitude of the feat make this a very motivational read.
But don't take my word for it. Check out this post from the Happy Runner blog -- http://tinyurl.com/yhmjsep. She gives it the same endorsement!