Another method to locate additional books is to check in Books in Print and British Books in Print. All libraries have at least one if not both. These books can also be requested through interlibrary loan or ordered from your local bookstore. Additionally, Books in Print will list the publishers' address for a particular book, and you can purchase it directly from the publisher. While doing you research, don't forget the magazines.
Many libraries subscribe to a large number of publications that are useful.
Also, most libraries have the Reader's Guide to Periodical Literature. This is the magazine equivalent to the Book in Print listing. Magazine articles can be ordered through interlibrary loan just like books, but there may be a photocopy fee. If there is, you get to keep the photocopies. References to a dig are often noted in books.