The mechanism of cancer is a lengthy and complicated process. Uncontrolled cell division, delay in ageing, problem in maturation due to activation of inactivated oncogenes cause cancer. If you want to learn the mechanism kindly see the links given by karogi sir and Robert sir above. If you want to read book then in my knowledge the topic neoplasia from robins text book of pathology will clear all fundamental doubts.
Thanks everyone, I'm glad to continue reading new delvelopments in research involve cancer though it is not just complicated but also a mystery at some point.