I recommend that you give away unlimited free copies of the entire book in PDF format. You say that you've already milked as much money as you could out of the print edition, which wasn't much. I've studied this stuff for years as a librarian and an anati-copyright activist. People are way to cautious when it comes to distribution of their intellectual works. If the book had been published recently, it would be smarter to just release a few chapters. Of course, many periodicals embargo the posting of articles online, simply because you want people to buy the print magazine. I've done that with Practical Anarchy magazine and we did it with content from Alternative Press Review.
I understand that it is difficult to figure out the impact of free online distribution of your content. I was working in the circulation department of Science magazine back in the late 1990s when we grappled with how to make Science available online. There was plenty of worry within AAAS about this. We watched Nature closely, which took fewer risks than we did.
There are some good reasons why you should make your book available online. The first is the Grateful Dead effect. Sharing promotes your product. The more people who discover your book are potential fans who will buy future books when you publish them. People will spread the PDF of your book around, thus increasing the circulation of your name and the content of the book. Most people will not read the entire PDF--studies have found that people won't read the entire text of books online. But they will read some of it and will either be motivated to buy copies of the book or will eagerly await your new books and articles. This will also help you increase your radio audience and crowds at public appearances.
Chomsky is a big name in part because his works are widely available online, both in text and audio form. This is one of the ways in which open sharing of content (also known as piracy) helps build up your audience. Microsoft spends millions of dollars trying to prevent piracy, but the ironic thing is that their empire is built on piracy. If office geeks like myself weren't putting illegal copies of Windows on computers back in 1991, then Microsoft would have had a harder time exploiting the built up fan base.
You really can't lose and it's likely that being more liberal about online distribution will pay off down the road.
I have my own paper copy. ;-)
Chuck Infoshop.org