Final answer:
The given source is a book, as indicated by the inclusion of an edition number, the city of publication, the publisher's abbreviation, and the year of publication in the citation.
Step-by-step explanation:
The source represented in the provided example, 'The Chicago Manual of Style. 15th ed. Chicago: U of Chicago P, 2003.' is a book. This can be determined based on the citation style, which includes an edition number (indicative of a book rather than a periodical), the city of publication, the publishing house's abbreviation (U of Chicago P is short for University of Chicago Press), and the publication year.
These elements are traditionally associated with book citations, particularly in styles such as MLA, APA, and the eponymous Chicago Manual of Style (CMS).