Every library book today has subject headings assigned to classify it. Sometimes using key words will find you books on your subject, but except for very specific searches with distinctive words, using subject headings (known as LCSH's, or Library of Congress subject headings) will usually retrieve more books than just a keyword search. Use key words to get LCSH's for better results.
Also, remember to take your searches, if you like, from the local to the national/international level, by using Worldcat. If we have four or five books on subject x, and you want or need more information, search Worldcat using the same LCSH's you used for our catalog, and if you find books we don't own, you can order them by using Interlibrary loan (ILL).