People are frequently surprised that my book, Higher-Order Perl, is available as a free download from my web site. They ask if it spoiled my sales, or if it was hard to convince the publisher. No and no.
I sent the HOP proposal to five publishers, expecting that two or three would turn it down, and that I would pick from the remaining two or three, but somewhat to my dismay, all five offered to publish it, and I had to decide who.
One of the five publishers was Morgan Kaufmann. I had never heard of Morgan Kaufmann, but one day around 2002 I was reading the web site of Philip Greenspun. Greenspun was incredibly grouchy. He found fault with everything. But he had nothing but praise for Morgan Kaufmann. I thought that if Morgan Kaufmann had pleased Greenspun, who was nearly impossible to please, then they must be really good, so I sent them the proposal. (They eventually published the book, and did a superb job; I have never regretted choosing them.)
But not only Morgan Kaufmann but four other publishers had offered to publish the book. So I asked a number of people for advice. I happened to be in London one week and Greenspun was giving a talk there, which I went to see. After the talk I introduced myself and asked for his advice about picking the publisher.
Greenspun reiterated his support for Morgan Kaufmann, but added that the publisher was not important. Instead, he said, I should make sure to negotiate permission to make the book available for free on my web site. He told me that compared with the effort that you put into the book, the money you get back is insignificant. So if you write a book it should not be because you want to make a lot of money from it but because you have an idea that you want to present to the world. And as an author, you owe it to yourself to get your idea in front of as many people as possible. By putting the book in your web site, you make it available to many people who would not otherwise have access to it: poor people, high school students, people in developing countries, and so on.
I thought that Greenspun's idea made sense; I wanted my ideas about programming to get to as many people as possible. Also, demanding that I make the book available on my web site for free seemed like a good way to narrow down the five publishers to two or three.
The first part of that plan worked out well. The second part not so well: all five publishers agreed. Some agreed reluctantly and some agreed willingly, but they all agreed. Eventually I had the book published by Morgan Kaufmann, and after a delay that seemed long at the time but in retrospect seems not so long, I put the book on my web site. It has been downloaded many times. (It's hard to say how many, since browsers often download just the portion of the PDF file that they need to display.)
Would the book have made more money if it were not available as a free download? We can't know for sure, but I don't think so. The book has always sold well, and has made a significant amount of money for me and for Morgan Kaufmann. The amount I made is small compared to the amount of work I had to put in, just as Greenspun said, but it was nothing to sneeze at either. Even now, ten years later, it is still selling and I still get a royalty check every six months. For my book to have lasted ten years is extremely rare. Most computer books disappear without a trace after six months.
Part of this is that it's an unusually good book. But I think the longevity is partly because it is available as a free download. Imagine that person A asks a question on an Internet forum, and person B says that HOP has a section that could help with the question. If B wants to follow up, they now must find a copy of HOP. If the book is out of print, this can be difficult. It may not be in the library; it almost certainly isn't in the bookstore. Used copies may be available, but you have to order them and have them shipped, and if you don't like it once it arrives, you are stuck with it. The barrier is just too high to be convenient. But since HOP is available on my web site, A can include a link, or B can find it with an easy web search. The barrier is gone! And now I have another reader who might mention it to someone else, and they might even buy a copy. Instead of drifting away into obscurity, HOP is a book that people can recommend over and over.
So my conclusion is, Greenspun's advice was exactly correct. As an author, you owe it to yourself to make your book available to as many people as possible. And the publisher may agree, so be sure to ask.
[ Addendum: Some people are just getting the news, but the book was published in 2005, and has been available as a free download since 2008. ]