The southern (Unterland) half of the old Burgraviate of Nuremberg, held as a free imperial principality by a Franconian cadet line of the Hohenzollerns; its margraves ran their own army - hired out to Britain for the American War - until the last of them sold the state to Prussia.
Sold outright: on 2 December 1791 the childless Margrave Charles Alexander ceded sovereignty over Ansbach (and Bayreuth) to his kinsman King Frederick William II of Prussia in exchange for a life annuity and retired to England; formally annexed by Prussia on 28 January 1792, then transferred by Napoleon to Bavaria in 1806, which the Congress of Vienna confirmed in 1815.
Beautiful Lady Craven: the Original Memoirs of Elizabeth, Baroness Craven, afterwards Margravine of AnspachElizabeth Craven, ed. A. M. Broadley & Lewis Melville · 1914
First-hand memoir from inside the Ansbach court by the woman who married Margrave Charles Alexander and left for England with him after the 1791 sale - the polity-specific English source for its final decades.
Iron Kingdom: The Rise and Downfall of Prussia, 1600-1947Christopher Clark · 2006
Explains the Hohenzollern dynastic system that made Ansbach an appanage and the Prussian appetite that swallowed it in 1791.