Library books are cataloged by title, author, publisher, date of publication, and subject heading. A subject heading is an assigned term that tells you something of what the book is about. 

For example, you might come up with the keyword "number theory". Relevant books might be cataloged with the subject headings "Number theory", or broken down into "Number theory -- Problems, exercises" or even "Algebraic number theory". You might even, as in the example here, see subject headings in other languages. Knowing these subject headings can make your catalog searches much more effective. But how do you find them? 

1. Start by brainstorming keywords. This only takes a few minutes but can save you tons of time later. Think about your topic, and write down every word you can think of related to that topic.
2. Then, go to the library catalog and start searching. You'll find at least a few books related to your topic.
3. Look at their catalog records and take note of the subject headings listed under the item details:

You can even click these subject headings to find more books cataloged in the same way.