Richard Dawkins' arguments against religion are well known. The God Delusion
is a collection of such arguments, with answers to many of his critics. Some people object to Dawkins' stance on religion, but he rejects the idea that arguing about religion is OK as long as you keep away from a list of taboos which you are not allowed to mention - this is strictly no-holds-barred. Thus he shows how horrible the message from religious scriptures would be if you took it all seriously, as well as speculating on how religious belief could have originated in the first place.
Dawkins claims that the book is an argument against religion in general, but I found that actually his arguments were chiefly against its more irrational forms. Arguments against those with more moderate beliefs seemed weak to me - the only one of substance being the effective support they give to the more extreme proponents. So if you are an atheist then you probably enjoy Dawkins arguments, but I feel that it is probably more important for those of you of a religious frame of mind to read it, to check whether you really identify yourself with the more ridiculous things done in the name of religion.