Writing a book on spec is very different from commissioned work because there is no feedback. You’re out there on your own. But there are two sites I’ve discovered that address the problem.
Litopia is run by literary agent, Peter Cox of Redhammer Management Ltd. You send a writing sample to Litopia, and, if the moderators like your work, you get to join the site. You do some crits of other people’s work, post your own, and start assimilating the response . Litopia is at present rejigging a system whereby your work can rise through the slushpile and get pitched to The Agent himself. It has the advantage of fostering long-term relationships between writers who can get thoroughly familiar with each others aims and ambitions.
YouWriteOn is backed by the British Council. There, you post your work, and, as at Litopia, earn crits by doing crits. The difference is, it’s a much bigger pond, members award marks to each other, and the top five chapters each month get feedback from industry professionals.
Both sites have seen member writers find representation and publication, and both are free.