That's easier said than done. But if you cannot find a suitable banner exchange for your field or topic, then think about creating one. This gives your site greater visibility, increases its link popularity, and gives the search engines more links to follow back to your site. Another form of free content is to provide code for people to place search boxes on their Web sites. Again, this will increase the exposure your Web site gets and the number of links back to your Web site.
Authors can also write book reviews and create book lists for other sites.
If possible, ask the other site to mention your book. If people just see the book and author's name, a URL isn't absolutely necessary. They will now have something to search for if they are curious about you and what you have published. Create a Webring. Many people misunderstand the benefits of webrings.