This game, known to most card game books as Yablon, is also commonly called Ace-Deuce, In Between or Between the Sheets. American casinos have recently begun to call it Red Dog, which is confusing, since since Red Dog was formerly the name of a different game.
Yablon is a banking game in which two cards are dealt face up, and the player bets on whether a third card will rank between the first two cards. Detailed rules can be found in the FAQ of the newsgroup rec.gambling.misc, where it is listed under the name Red Dog.
The Free Red Dog site also has rules for the casino version of the game, and a Flash Red Dog program which visitors can play on line for fun.
The Casino-info.com site has a page of Red Dog rules.
Roland Scheicher has written an article on Red Dog for the German Wikipedia.