Delshad: Welcome back! I'm glad you decided to stick around.
I know it is often difficult to get access to actual books, so I'll devote this post to free, on-line resources I consider to be very good general references:
actionbioscience.org website contains a collection of "popular science" articles on evolution written by some of the top scientists in the field.
Evolution and the Fossil Record, the American Geological Institute's excellent "evolution primer".
Origin of Species on-line. To be honest, I found Darwin's book very hard going. This on-line version can help if you want to see what the old man actually wrote. As Mammuthus noted, it is extremely detailed, and can be heavy reading. However, if you're looking for a description of every observation Darwin made in coming up with his theory, this is the place.
Anyway, if you have any questions on any of these articles, please don't hesitate to post them.