Throughout his life, Thomas Jefferson owned between 9,000 and 10,000 books. By contrast, the Gering library contains around 32,000 books. Jefferson sold his personal library to replace the destroyed original Library of Congress when the British burned it during the War of 1812. This was not his first library though. In 1770, his original library of 400 books was destroyed in a fire. These were books he inherited from his father and others he acquired when he was in college. Jefferson began rebuilding his library immediately, adding 2000 books he bought while in France and nearly 700 he had inherited from a friend. By 1814 he had around 6,500 books which he sold to rebuild the Library of Congress. When Jefferson died in 1826 he owned around 1600 books. Most of these were sold to pay his debtors.

How did Jefferson keep track of all these books? The Dewey Decimal system wasn't used until 1873.

The philosopher Francis Bacon determined there were three kinds of knowledge; Memory, Reason and Imagination. Jefferson modified this theory into Memory, Philosophy and Fine Arts. He used these three categories to organize his books. From the three main categories, he further divided them into 44 chapters. From there the books were organized chronologically or analytically according to Jefferson's inclination.

Under the Memory section Jefferson included books on history, agriculture, chemistry, medicine, zoology, and what he called "Occupations of Man. Technical Arts."

Under Philosophy Jefferson listed ethics, law, politics, mathematics, mechanics, astronomy and geography. This is also where he placed his copy of the Qur'an and his Hebrew Bible.

Under Fine Arts Jefferson included books on architecture, gardening, painting, sculpture, music, poetry, fiction, and what he called criticism.

He sent a carefully organized list with the books to Washington. When the librarian put them on the shelf, he retained the categories and chapters, but alphabetized the titles within each chapter. Jefferson was not impressed, but he eventually agreed that others would be able to use the library more easily if it was organized alphabetically.

In 1897 Charles Martel set out to better organize the now one million books in the library. By 1939 the Library of Congress Classification system was mostly complete. Many colleges and universities use "LC," as they call it, to organize their libraries. However, most public libraries use the Dewey Decimal System.

Rather than three main categories, the Library of Congress now separates books into 21 categories, using the alphabet to distinguish each category. An additional letter is assigned for sub-categories.  For example, Agriculture is now filed under S. Horticulture is SB, animal husbandry is SF and hunting is SK. From there, Martel used numbers to further separate out each subject. Continuing with hunting, you would find fox hunting under SK 284-287 and wildlife management books under SK 351-579.

Fiction is separated out by the author's origin, so Literature is P but English literature is PR and American literature is found under PS. 

The Library of Congress helpfully assigns their own subjects to most books so that libraries don't have to start from scratch when they add a new book to the collection. If you look at the back of the title page in any book, you will likely find something like this: "1. Pickett, Joe (fictitious character)- Fiction. 2. Game wardens- Fiction. 3. Wyoming- Fiction. Then after that you will see something like PS3552.O87658T76." The first part are the subjects the Library of Congress assigned, which many libraries use. The long number is where the book would be shelved. PS being American Fiction. As you can see, a lot goes into deciding where something would go on the shelf using Library of Congress classification, but I think once you learn the system it would make perfect sense.