Depends on the category:
1- Spiritual: I'd suggest the Quran, the Bible or the Torah (depending on your orientation :) This will give you the other side of the story and counter-balance Razan's suggestions :D
2- Motivational: I love 'Think and Grow Rich.' It's a very old book, goes back to 1920s, but still going strong.
3- Politics: Mein Kampf by Adolf Hitler. The world didn't really change in more than 70 years.
4- Arabic novels: Everything I read for Amin Ma'louf and Abdulrahman Muneef was great.
5- Business: Good to Great
6- Investment: The Intelligent Investor, another oldie and considered the best book on investment by the best investor in the world, Warren Buffett.
7- Time management: Getting Things Done. a perfect fit people who like processes to get their time properly managed. Not an easy read through. Requires practice.
Good luck