Jonathan Zittrain, co-author of an excellent study of stock spam, has just released a book called The Future of the Internet - And How to Stop It, available both on paper or as a Creative Commons-licensed download.

The book isn't primarily about spam, but any discussion of the future of the Internet — which Zittrain sees as potentially bleak, by the way — must necessarily cover the topic. Zittrain outlines the problem and then talks about responses to the problem in the form of open collaborative grassroots projects. I haven't had time to do more than skim it, but it looks like it might be worth a read, if only as a possible source of new ideas and a way to look at spam as an instance of the larger problems facing the Internet.