Apostasy means "renouncing the faith." If someone is a Jew or a Hindu or whatever and they decide they don't want to be one anymore, that is apostasy.
The Qur'an says apostasy from Islam is a crime punishable by death. In Islamic states, this is reinforced by law. You cannot convert out of Islam. Once you're in, you're in for good.
It takes a great deal of courage to leave Islam, and the book, Leaving Islam: Apostates Speak Out is a collection of first-hand accounts of what happens when a person renounces their belief in Islam. It'll give you an inside view of what it's like to be a Muslim. It makes for some surprising, eye-opening reading.
You can find out, in Leaving Islam, the many different ways Muslims around the world deal with the difficult decision to renounce the religion.
Probably the only way we're going to get a straight story about what Islam is like is by listening to apostates. They have been Muslims and know all about it, and yet they are not apologists for it. They aren't trying to sell you on it. And all the apostates have a very consistent point of view on Islam.
Of course, they are all against the religion's intolerance of apostates, but many of them do not hate Islam itself. They just dislike the fanatics within the religion that want to keep to the old ways. Islam has a kind of civil war going on within it right now, the old-ways fanatics against the more numerous Muslims who would like to modernize their beliefs. Read more about that civil war in The Short Answer.
Leaving Islam: Apostates Speak Out is a good read but it's also a great reference book. Appendix A is a large list of quotes showing the violence, hatred, and intolerance commanded by the Koran. These are what the fanatics believe in and these are the main reasons the apostates left the religion.
Appendix B is a list of web sites, organizations, and email addresses of groups who are trying to help create a secularization of Islamic societies (that would go a long way toward curbing the development of Islamic terrorists) and groups that promote freedom of thought and freedom of religion in general. Those of us committed to defeating terrorism would do well to support those groups.