Bluebook Rule 15 governs most materials not published on a regular basis including "books, treatises, reports, white papers, dictionaries, [and] encyclopedias." In contrast, sources published on a recurring basis such as law reviews, journals, newspapers, and magazines are governed by Rule 16.
Shorter works published in a nonperiodic source, such as chapters in a book, are cited using Rule 15.5.
There are three main places to search for books:
Older books that are no longer under copyright may be available digitally through Google Books or HathiTrust.
Many online sources with print characteristics could be classified as a report or whitepaper. Under Bluebook Rule 18.2.1(b)(ii) these online sources with print characteristics should be cited according to Rule 15 with the URL appended to the end. Common features of this type of online nonperiodic materials include:
These features are not required or exhaustive.
You may need to locate the landing page on the host website for an online nonperiodic source to find all the necessary information to make a citation.