Mark Gregory Pegg,
The Corruption of Angels: The Great Inquisition of 1245-1246.
Sample with specific information. Notes that the figure and description of the indiscriminate killing appeared first in a letter addressed to Innocent III from Arnaud-Amaury himself.
Information page for the Languedoc region. Citation at bottom of page, for
Patrologia Latinae Cursus Completus, series Latina, 221 vols., ed. J-P Migne (1844-64), Paris, Vol. 216:col 139.
Crusades-Encyclopedia notes that estimates of 10 to 20 thousand deaths were given after the siege, including by Catholic sources. Beziers had a population around nine thousand at the time, and only a few hundred were Cathars.