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Leanpub is close to what you want. You can easily get started and charge your earlier supporters less than later supporters, as the book gets closer to completion.

I wrote a couple of blog posts about the process: http://www.mooreds.com/wordpress/archives/1339

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Thanks! Did you already have some audience to start with (i.e. blog readers, Twitter followers, etc) or were you starting from scratch?

I wrote a series of blog posts, was active on some newsgroups, and reached for it to some influencers, but, no, I didn't have an audience. I think the marketing post is linked in the post I mentioned, and that has some more details.

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