The movie is loosely based on the 1872 Joseph Sheridan le Fanu book Carmilla, which can be read online at many different sites. You can have it read to you here courtesy of Librivox.
The New York Times calls it an "exquisitely mounted reprise of the old human vampire theme". Moria gives it 2 1/2 out of 5 stars. DVD Talk calls it "One of the most coveted but elusive Euro-horror delights from the 'first wave' of the late 1950s and early '60s"