According to the English philosopher John Lucas, philosophical naturalism is now the orthodoxy of the Western intellectual world. This is plausible; it is at any rate one of the current academic orthodoxies (another, perhaps, is the sort of creative anti-realism and relativism with respect to truth associated with certain brands of post modernism). Perhaps the easiest way to understand naturalism to see it as the view that there no such person as God (no all powerful, all knowing and wholly good person who has created the world and has created human beings in his image), nor anything at all like God. The naturalist--the contemporary naturalist, at any rate--typically adds a high view of science, seeing it as the only possible source of our salvation.
Daniel Dennett's Darwin's Dangerous Idea is a big (very big), bright exploration and defense of naturalism--or at least of one aspect of it. In several areas it is authoritative; it is written with passion and power; I wouldn't be at all surprised if this book acquires the status of a minor (or maybe major) classic among statements of naturalism. Dennett tries to do at least three things: (1) explain Darwin's dangerous idea and show how the world looks if you take it really seriously, (2) argue for this idea, or perhaps defend it, or perhaps argue that it is at any rate possibly true, or perhaps persuade us that it is true, or possibly true (it is hard to tell which), and (3) buck up and admonish timid, half-hearted naturalists who are unwilling to accept the full implications of their position, thus falling into false consciousness.