free. I would have posted it online for free, and ask people to donate as they see fit. You would be amazed at how generous people could be, if they felt that they are being given something with value for FREE.
I would easily pay $1 for the book, but I would pay more as a donation... if it was a good read.
And if I thought it was really good I'd send the full $10 and post a detailed review.
You really think such an endeavor would be more profitable if set to free? You think it a good idea to lean on donations?
Based on what I have seen thus far, human behaviour treats something they have paid for as more valuable = more people will actually read it if they have given something up for it.