What is the best book to learn probability? Question is quite straight... I'm not very good in this subject but need to understand at a good level.

I happened to take an introductory course on probability and statistics on two different universities. In one they used a horrible book, and in the other they used a truly amazing one. It's rare that a book really stands out as fantastic, but it did.
Probability and Statistics for Engineers and Scientists
by Ronald E. Walpole, Raymond Myers, Sharon L. Myers and Keying E. Ye. 
The version number doesn't matter, just find an old version second hand. 
It is really thorough, takes one definition at a time, and builds on top of that. The structuring and writing is top class, and the examples are well chosen.  
Don't worry if you are not an engineer. When using examples they have taken them from the domain of engineering, eg "A factory produces so and so many items per hour, and only so and so many can be broken, ...", instead of using examples from fx social science or economics. But they don't involve engineering science such as statics, aerodynamics, electronics, thermodynamics or any such things. This means that everyone can understand the book, it does not even help to have an engineering background. Perhaps the examples are more appealing/interesting to engineers, but that's all. 
