Phantom is working on the spec. right now,  and I have posted a wiki on assets (which includes information on issuing assets, trading on the DEX, paying dividends, etc.): https://wiki.counterparty.co/w/Assets.

The wiki on broadcasts and bets is undergoing another revision right now and will be up shortly. Between these two wikis, Counterparty's basic functionality ought to be covered.

The wikis have had to undergo several revisions in order to become more user-friendly, and any criticisms/suggestions are greatly appreciated, and, for the assets wiki, can be posted here: https://forums.counterparty.co/index.php/topic,142.0.html.

EDIT: Note that the section on trades involving BTC is quite short right now because we are currently considering whether and how to revise it. For those who are interested in the discussion, please see the thread on the official forums: https://forums.counterparty.co/index.php/topic,132.0.html.