Here you are, the promised plugin. It searches for author. If you'd rather search for books just open the file in notepad and change the value="author" to value="book" and save.
Copy the xml file to "c:\Program Files\Mozilla Firefox\searchplugins\", that is if you haven't changed the default installation directory for FF. Restart Firefox and you're ready to go. I even added favicon, my own design.
Let me know how it works.