Just the Facts: How "Objectivity" Came to Define American Journalism by David T.Z. Mindich (Revised edition, 2000)Read about the other entries on the list.
This is a very approachable but authoritative biography of "objectivity" that explains how and why we value that notion and, by extension, why we feel so betrayed when objectivity is lost. Knowing this helps us see the arch and return to the topics that really matter, while fending off false claims we hear every day. Not bad for one book.
Also see Derek B. Miller's ten top books about the Iraq War.