Wilfrid Mellers, François Couperin and the French Classical Tradition, (Faber & Faber, 1950, rev. ed., 1987), remains the most extensive English-language treatment of Couperin. It is out of print, but locatable on Amazon and other sellers.
For more recent takes, see the following (both with some idiosyncracies and innacuracies):
David Tunley, François Couperin and 'the Perfection of Music,' (Ashgate, 2004). At 172 pages, it's an excellent introduction.
Philippe Beaussant, François Couperin, translated from the French by Alexandra Land, (Amadeus Press, 2003.) Out of print.