U_K did a pre-release of Ascension, wherein he let people pay for the book early, receive an unfinished copy, and he'd keep updating it as new work was done on it. He did a similar thing with the Bestiary prior to its release.
However, the pre-release for Ascension has been a disaster, in U_K's own words (he may have called it something different, but that was the sentiment). It's taken quite a long time even by U_K's standards (he always misses his own deadlines by months), and he's grown frustrated at the idea of having an incomplete book out so long. He's long since stopped taking pre-orders, but most of the people here already have the book (the latest beta version is 2.6).
Hopefully, it'll be completely done soon...