James Tanner gives us a good, albeit short, overview of what the Library of Congress has to offer genealogists. The statistics he provides are over-whelming. For example, there are a total of 141,847,810 items in the collections, including: 21,218,408 cataloged books in the Library of Congress classification system, 62,778,118 total manuscripts, 5,357,385 maps, 16,086,572 microforms, etc.
Perhaps most helpful is the link the author provides to the "Local History & Genealogy Reading Room." Check it out at: http://tinyurl.com/y8cajbw