Giving away a free book is fine if you have other books to sell. Me, I just have one, so that won't work. So I make the first 50% available at Smashwords and that's as generous as I can be.
Someone may read the first half and still think, "Yeah, but it could fall apart in the 2nd half." But you know, there's nothing I can do about that. "You want a guarantee, buy a faucet," as the saying goes.
I've paid thirty bucks for a book by an author I've liked, in a series I liked, and I ended up hating it, but that's how it goes sometimes.