P, or the priestly source, is one of the more easily defined pentateuchal sources in the tradition documentary hypothesis. Its texts are concerned with ritual, purity, law, and genealogy with one of its main constituents being the Holiness Code (H) in Leviticus 17-26. P was originally considered one of the earliest sources of the Pentateuch until Wellhausen nicely packaged the view of Graff, Reuss, and himself in his Prolegommena, and argued that the priestly laws were actually the latest strand. This has been criticized by some as a general tendency of viewing priestly elements as late corrupting accretions to pure beliefs. There has been debate on whether P was a coherent document to which JE were slotted in (Noth) or if P is merely a redactionarly layer (Cross). Milgrom, and other scholars like Hurvitz, Weinfeld have argued that P should dated to a similar period as J, and before D, and that it reflects actual ritual practice in the Solomonic temple. It has usually been assumed that H comes from the early exile, whereas P merely inserted itself into and edited H at a later period. The old consensus concerning the documentary hypothesis is long gone, however, things are up for grabs at the moment.
ABD V:454-461; Gordon J. Wenham, "Pondering the Pentateuch: The Search for a New Paradigm," in Arnold and Baker, The Face of Old Testament Studies: A Survey of Contemporary Approaches, p. 116-144 (See also similar article by same author)