Remember that Library of Congress Classification arranges materials by subjects.
The first two lines describe the subject of the book
E745 = Military History
This line represents the person who is the subject for this book.
.M3 = MacArthur, Douglas
This line represents the last name of the author
.M27 = Manchester, William
This is the year the book was published.
Why is this important?
Because books are classified by subject, you can often find several helpful books on the same shelf or nearby.
Since Library of Congress Classification arranges materials by subjects, knowing the letter(s) for your subject area gives you a place to start browsing the shelves. Which letters represent your subject? View the Library of Congress Classification Table.