Use the search box on the home page to search HOLLIS or HOLLIS Classic to locate books at any Harvard library. HOLLIS describes more than 14 million volumes. There is a link to HOLLIS in the top right-hand corner of all of the library's web pages.
See HOLLIS vs. HOLLIS Classic: Which One to Choose? for more information, and a video, about the two catalogs.
Note that some materials are located at the Harvard Depository, a large, off-campus storage facility. Many of these items can be requested online through the HOLLIS Catalogs; other items will need to be requested through the library that owns the item. Items are generally delivered and available within one business day of a request.
If you don't find what you are looking for at Harvard, see Books Beyond Harvard.