Andreas, The best way I’m aware of to do read posts with #WithKnown currently is to post a status update to the effect. I’ve been considering doing a plugin similar to Jonathan LaCour’s Watching but for books. In fact, other than some cut and paste work and changing the microformats, it should be a simple thing. I’d almost feel guilty for “stealing” it.
I’ve started hacking away at a Read plugin similar to the bookmark or like functionalities that simply bookmarks articles which I’ve read online, but it’s got some quirky upstream issues that need to be solved before I can finish it. Ultimately it’s really for online content more so than books (or even magazines) though. I may circle back around to it sometime toward the end of the summer as I’m trying to finish up some last pieces on my Reading workflows for WordPress.