I have a book I published using Leanpub and I set the minimum to be $10. Some people pay the minimum. Some people pay the suggested price of $15. Some have even been as generous as to pay MORE than the suggested price.
If I ever decide to give away my book for free for a limited time, I would not feel bad about people giving me $0 when they could give me whatever they want.
I am in agreement with Raganwald in that I just want people to read it and find it helpful.
I wrote a short Python tutorial (http://www.korokithakis.net/tutorials/python/) which I packaged up as a book in a few hours and put on LeanPub. I used to sell it for $2, but the LeanPub guys changed the pricing structure to minimum/recommended, so I set the recommended price at $5.
People have been buying it for $5 ever since, and I've even had some $8s and $10s. I even include a link to the tutorial right on the book page, because I don't think it's worth even $2, but apparently people do!
If I ever decide to give away my book for free for a limited time, I would not feel bad about people giving me $0 when they could give me whatever they want.
I am in agreement with Raganwald in that I just want people to read it and find it helpful.