Sources are extremely important to students of history because they speak directly to the credibility of their own work.
There is a fundamental distinction between works of scholarship and journalistic or populist histories. Scholarly studies, by definition, are replete with footnotes citing the source or sources of information that the author used in writing his or her history. The more credible those sources, the more credible the final product. There are also distinctions in types of sources. "Primary" sources are always the goal to a serious student of history and include documents produced at the time being studied by witnesses to the events depicted and interviews with participants in the events described -- obviously only possible with current history.
"Secondary sources, in contrast, involve second or third person depictions of the events being described, for example, newspaper articles, some biographies, and so on. Drawing from other scholar's history books is secondary research, unless the researcher makes the effort to locate and use the original document cited in the book.
There is a grey area of scholarship frequently employed by prominent journalists like Bob Woodward. There is no question that the preparation of such books involved many first-person interviews, but these books do not qualify as "scholarly" because they don't cite the sources. Readers are left to ponder the identity of the source and, consequently, its credibility and veracity.
The most frequently cited example of the distinction between scholarship and populist history -- a study that may raise important points, but that is lacking in scholarship -- is the late Howard Zinn's "A People's History of the United States." Zinn's book is a classic among the American left for its depiction of the negative side of America's birth and growth as a nation, for example, its discussion of the World War II era treatment of Japanese-Americans by the U.S. Government.
Zinn's book, however, has been widely condemned by liberal historians because of its lack of scholarship. The oft-cited quote from the late liberal historian Arthur Schlesinger sums of the feeling of many scholars on the left and the right: "I don't take him [Zinn] very seriously. He's a polemicist, not a historian." Michael Kazin, a historian of the far-left, referred to Zinn's book as "bad history, albeit glided with virtuous intentions."