Legal Question in Intellectual Property in California

This really depends on the exact terms of your deal with the Government. If your writing was "work made for hire", then the work 'belongs' to the government, and is therefore not protected under copyright law, so you probably won't be able to do anything about this third party use.

If it's NOT "Work made for hire", then YOU own the copyright, and probably CAN stop the publication by this other outfit.

Your best bet is to have a lawyer review the contracts and advise you. The good news, this shouldn't be a very lengthy or expensive consultation. (Maybe even free)

If YOU own the copyright, and want to stop this publication, you'll probably need to sue the infringer. "cease and desist" letters sometimes work, but they're usually just a prelude to a lawsuit.

I certainly can't give you a better answer, but I'd like to add one observation. A couple decades ago, the United States Postal Service began placing a copyright notice - the familiar "circle-C" - on the margins of sheets of stamps. Evidently, it was concerned about - and able to - protect the stamp designs done for it by the artists it hires. This may indicate that a unit of the government is not prevented from acquiring, holding and enforcing a copyright. In the USPS case, their concern was truly prevention of design copying, and not counterfeiting of stamps. It's possible, of course, that because the USPS (like Amtrak) is in some way a separate entity or independent agency, it has different copyright rights than the basic Federal government. Just a thought.

Bryan is correct on two fronts. Although copyright does not protect works AUTHORED by the federal government, the government CAN and DOES acquire copyrights by other means, and can enforce them. I also believe that because the USPS is not a government agency, the works it creates ARE protected by copyright law (though some more research would be required to confirm this)