The only real "ghost town" in NH in the sense of a full self-governing entity (= Town, which is different than villages) was Livermore, which was incorporated in July 1876 and disincorporated in August 1951. Both were by act of the NH legislature... in 1951 there wasn't anyone living there anymore. Some of this is in Fran Belcher's "Logging RR's" which was referenced above by Dr. Wu.
I know there are some other lost villages (I believe Monson Village in Milford/Amherst? and Landaff has something where a village disappeared, also most/all the logging camps on the old USGS maps & in Belcher's book) but these are just villages w/o any kind of a government. Livermore had an interesting history, again I think it was referenced in Belcher's book, basically there were "voting wars" between the handful of residents in the Saunders logging company vs. those in the Henry logging company who owned the western half of the town. (I'm not telling this right & really should find a proper reference. It's either in Belcher's book or George F. Morris' autobiography "Reminiscences of a Yankee Jurist", which Belcher quotes extensively, and is an excellent read in its own right, though out of print.) The last residents left sometime in the 1930's - 1940's after most of the town was acquired by the USFS, small pieces of it were (and I think at least one still is!) private property until recently (e.g. 1980-2000, there were some WMNF acquisitions in Livermore.)
I've never tried snooping around to see if there are still ruined buildings around, though I've heard they're there.
I've tried to get more info on Livermore, have found a few leads in the NH state archives & NH state library, but my interest in unincorporated townships has been at the bottom of my priority totem pole in recent years. If anyone finds anything interesting, please post (or PM me) ... please cite references if at all possible.
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GorpBurp beat me to it with some of this.
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