Charles Darwin wrote a book which was published in 1859. It's obsolete but it's still a masterpiece. You can read it for free online.

On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life

Please notice Christian-evolution-deniers that "races" in Origin of Species had absolutely nothing to do with human races. Darwin didn't publish anything about human evolution until 1871 because he thought that might get in the way of the public being able to accept his natural selection idea. In 1871 Darwin thought the human race was ready for the truth about their species and that's when The Descent of Man was published.

Personally I prefer more up to date books about evolution because there's been a tremendous amount of progress in evolutionary biology since 1859, and that's why I highly recommend Why Evolution is True by Jerry Coyne which was published in 2009. It costs 18 dollars but if you don't have that much money you can at least read the customer reviews at amazon.com and/or read quotes I stole from Jerry Coyne's book and from his blog at Some stuff I stole from Jerry Coyne's Why Evolution is True.