If the person in question has been active for many years, it is likely that books have been written about him or her. This may be a novelist, a politician, or a scientist. Remember that when a book is written about a person, a Library of Congress subject category for that person's name is created. So using the Subject search in the catalog is a good way to find books about that person. For example, here is a screen shot of the results of a Subject search for books about Theodore Roosevelt:
You will see that there are even sub-categories, which will help you refine your search for books on certain aspects of Roosevelt.
Biographical Reference Sources
Many reference sources are excellent for finding biographical information. Here are a few:
American National Biography is a fantastic source for biographical information about Americans. It has over 127,000 entries, all full text, online. You can search the entries by a specific name, or by category, era, sex, birth date, and many other ways.
Complete Dictionary of Scientific Biography- This guide is excellent for finding biographical information about scientists. The entries are usually 3-6 pages long, are signed entries, and include bibliographies that will point you to more sources about the scientist.
If you are researching a literary figure, try the Literary Reference Center. This online database has full text biographical information, as well as articles on literary criticism and even original works.