To find the book number of a book online, you can follow the following steps: 1 Search online libraries: Many online libraries offer free e-books. You can use a search engine to find the website of these libraries and use the keyword "book number" to find the books of interest. Some common online libraries include the Google Library, the Public Library Union, and the National Library of America. 2. Search for the copyright information of e-books: Many e-book vendors (such as Amazon Kindle, Google Play Books, Apple iBooks, etc.) provide copyright information of e-books. You can search for the copyright information of books on these platforms to obtain the book number of the book. 3. Search for information about the book's publishing company: If you know the author and the publishing company of the book you want to read, you can search for information about the book on their website, including the book number of the book. Some of the websites of the publishing companies include: Amazon, Let the Reader Read (Let the Reader Read), and the Pub. 4. Use online book number inquiry tools: There are also online tools that can help you find the book number of a book. These tools usually require some payment to be purchased but can provide more accurate information. Some common online book number search tools included: Book Title Search website, Book Title Search Engine, etc. It is important to note that different books may have different book number format, so you may need to use different search tools to find the book you want.