In a library, each volume in a given collection will be assigned a call number by the cataloging department. This number is part of an intricate system of organization by means of which librarians and patrons can find the books they need. Each book's call number is based on its subject matter or genre. Thus, when books are shelved in order, you get large sections of related items placed together in a convenient manner. I would postulate that the term came from "call up", since one calls up the call number in order to locate the book.

There are two major cataloging systems in use in the United States. The first, the Dewey Decimal System, is probably the means by which the books were shelved in your grade school's one-room library. It assigns each volume a number from 000 to 999. This system is generally regarded as inferior, since it cannot encompass multiple subject headings or large collections very well. Therefore, in large academic libraries, you will usually find volumes shelved instead by the Library of Congress system. This system uses letters plus large strings of numbers to categorize each book. Since it is far more complex, it can encompass a huge and diverse collection. So, there are multiple types of call numbers used for multiple types of libraries, and the so-called numbers are not necessarily done only in numerals.

You can find the call number of a book by consulting a library's card catalogue, which at this point is far more likely to be a computer catalogue than an actual varnished piece of furniture with thousands of cards listing thousands of books in its thousands of drawers. You will need to know your book's citation in order to do this effectively: if you are not sure of the author's name, and the book you need is called Psychology, you probably have a problem. Ask a librarian and they may be able to help you. (Conversely, you may just piss them off, but they will try to be nice about it anyway.) However, since volumes are shelved by subject, you can use the call numbers of other books on the same subject to locate other sources. Just find several books with similar numbers and look around them on the shelves.