Bungie's next game after Reach will be "a new IP"; no more Halo from them. Microsoft's 343 Industries will keep expanding the Halo universe indefinitely; they claim to have storylines planned out from now til at least 2015, including possibly future games. Book 3 of the Forerunner Trilogy won't be out til at least 2012.
Reach is followed by Halo 1 chronologically.
Also, after Halo Reach comes Halo 4, the continuation of Master Chief's Journeys. As seen from the Halo 3 Legendary Ending, and the Halo 4 Teaser Trailer, Master Chief and the Frigate is heading to a Planet.