As far as I know, Riker was still the 1st officer when TNG ended...
I guess you're talking about some not so official continuation to the series?
Riker was slated to take command of the U.S.S. Titan at the end of Star Trek: Nemesis.
PICARD: Come. ...Will!
RIKER: Permission to disembark, sir.
PICARD: Granted. ...So, where's the Titan off to?
RIKER: The Neutral Zone, sir. We're heading up the new task force. Apparently the Romulans are interested in talking.
PICARD: Well I can't think of a better man for the job. ...If I may, just a word of advice about your first command.
RIKER: Anything.
PICARD: When your first officer insists that you can't go on away missions,...
RIKER: ...ignore him. I intend to. ...Serving with you has been an honour.
PICARD: The honour was mine, Captain.
There are a series of non-canon novels that follow the adventures of the Titan.