Many Wikipedia articles have an International Standard Book Number (ISBN) alongside the bibliographical entry for each book. Clicking on the linked 10- or 13-digit number allows readers to automatically check to see if that particular book is available in a library catalogue, at WorldCat, at Google Books, at major booksellers, or at other websites throughout the world.
An example of a bibliographical entry as a reference or a suggestion for further reading would look like this:
Carr, Edward Hallett (1972). What is history? : the George Macauley Trevelyan lectures delivered in the University of Cambridge, January-March 1961. Harmondsworth; Ringwood: Penguin Books. ISBN 0-14-020652-3.
The ISBN might lack dashes between the digits, or it might contain spaces between the digits, but it still provides the link. Note that different editions of the same book may have different ISBNs and books published prior to the 1970s do not have ISBNs, unless subsequently reprinted.