By it's very nature, it's difficult to go to the bookstore and pick up a book on public domain music. If anything, the library is usually one's best bet to search for books with music dated before 1923. There are a few books on the subject that I was able to get from my local bookstore that I've found an invaluable reference. They're published by Dover who has done a lot to make public domain material accessible to the public. The books I own and would recommend are:
Links to public domain music resources are provided on this page. I have a mailing list for discussion of locating public domain tunes and using Open Source programs to share them. See http://groups.yahoo.com/group/pdsongs if you'd like to join in.
To search for more public domain music resources elsewhere on the web:
To an article about creating Music on your PC.