In Cold Blood expertly moves between first person and third person omniscient, choices that Truman Capote made based on the way certain information needed to be conveyed.
The majority of the book is told from the third person omniscient point of view. The book begins in this way, for example, as the reader learns all about Holcomb, Kansas and the Clutter family. This point of view is especially helpful in providing what appears to be objective information to the reader. Truman Capote talked about this book as a new type of book, a "nonfiction novel." This means that the book is nonfiction in nature, which is true, but the book is written in much the same way as a novel might be. Third person omniscient point of view tells the reader everything, much like a nonfiction article might do. In fact, Capote's "reportage," as he called it, was the same type of reporting he would have done for a newspaper article.
Parts of the book, however, are told in the first-person point of view. These sections help the reader to see something from a specific person's vantage point, much like a novel might ordinarily do. Additionally, it can help build suspense, because the reader is only offered a limited perspective in this point of view. For instance, when Nancy's friends are walking through the Clutter home, the reader does not know exactly what they will find, where, and in what condition. This use of first person, rather than the third person omniscient, effectively builds suspense in this episode that would have been difficult otherwise.
The result of shifting between these two points of view is that readers are given different pieces of information more effectively.