The classic works on the Documentary Hypothesis (DH) are Julius Wellhausen's Die Composition des Hexateuch und der historischen Bücher des Alten Testaments and Prolegomena to the History of [Ancient] Israel (the latter of which, incidentally, is available in English for free via google books). The theory is generally attributed to Karl Graf, but it was Wellhausen who brought it to the forefront of biblical scholarship.
The basic premise of the Prolegomena is the identification of certain contradictions throughout the Pentateuch that point toward a progression toward the "Judaism" of the Exilic and second Temple period. The arguments are very nuanced, but Wellhausen's attention to detail is staggering. Central to his argument is the apprent change of opinion regarding the centrality of Jerusalem for the worship of Yahweh. The earlier strata seem unconcerned by, for instance, Solomon (or Samuel, Elisha, etc.) making sacrifices to God outside of Jerusalem, which is strictly forbidden in the later D and P sources. Moreover, none of the above mentioned people were Levites who were the sole practitioners of the temple cult.
Biblical scholarship has come a long way since Wellhausen, but one would be remiss to toss out his work as passe or otherwise "disproven." The DH is broadly accepted by nearly every serious scholar of the Hebrew Bible (with modification and individual nuance, of course) for good reason.
Don't accept (or reject) the theory just because someone on the internet said so; read the first few chapters of the Prolegomena (with a Hebrew Bible next to you if you can)--the man was truly brilliant.
Post Script: Incidentally, Wellhausen was a devout Christian--keep that in mind when you read about his theories.