If you can't find a book at Walden Library, look for it online. You may be able to find it for free or see a preview: the table of contents, introduction, and even complete chapters. When full books are available, they tend to be old—out of copyright and in the public domain—but occasionally you can find recent books in their entirety.
If you find only a preview, it may be enough to help you determine if a book is useful for your research and worth finding at a local library or buying. Previews can also be useful if you're waiting for textbooks to arrive.
Exploring these collections—browsing by subject area, or searching by topic, keyword, or author—are also great ways of discovering books.
Google Books might have previews or full books available. You may find links to purchase books.
Search by book title, author, or topic. Depending on the book(s), you'll see different viewing options offered.
If there is a preview available, you will see Preview listed under the title in the search results.
Google Books has viewing option information and examples, along with a general how to guide:
Open Library attempts to have a record of every book ever published. When available, it links to full e-books.
Enter a book title, author, or topic in the search box to see what's available.
Get more information on how to use Open Library at their Help page:
All e-books from Project Gutenberg can be freely downloaded in formats such as ePub, Kindle, and HTML. The collection is mostly fiction and older books, but you can also find classical texts from theorists.
You can browse the book catalog on their site or use the search box to enter search terms.
Get more information on how to use Project Gutenberg from their how to page:
Find full books at HathiTrust that you can view on screen. Some may also be downloaded as PDFs.
You can search for books with a Full-text or Catalog search. Full text will search for your search terms inside the text of the books. Catalog lets you pick title, author, subject, etc from the All Fields drop-down next to the search box.
Find out more about using Hathi Trust on their Help page:
The Online Books Page from UPenn lists over one million free books.
You can search by title and author, browse by author, browse by title, and even browse by subject.
Find out more about using Online Books on their FAQ page.