There's a persistent story that during the Albigensian Crusade when the crusaders were besieging a town that contained a population of both heretics and Christians, the knights asked the Abbot how they should tell who was a heretic and who was Christian. Amalric supposedly replied: "Kill them all, the Lord will know his own".
Did he really? And what is a good book on the Albigensian Crusade generally?
That quote was attributed to Amalric by Caesarius of Heisterbach in the latter's Dialogus miraculorum (Dialogue on Miracles), which was written about ten years after the event. Importantly, this work was intended as a collection of edifying [exempla] (https://en.wikipedia.org/wiki/Exemplum). In other words, Caesarius was more concerned with making a moral or spiritual point than with portraying the crusade accurately.
None of the other sources for the crusade, including those by first-hand witnesses, have Amalric saying this.
I recommend Mark Gregory Pegg's A Most Holy War: The Albigensian Crusade and the Battle for Christendom. On Amalric's supposed words Pegg says, "Whatever the truth of this anecdote--and there is only the report of Caesarius--it nevertheless lauded a homicidal ethic that, while shifting and changing through the decades, was an essential principle of the holy war on heresy."
The quote is apocryphal. I've written on this quite a bit as it turns out, as it is part what I research (heresy in 13th century southern France)
This is a discussion of the massacre at Beziers and the famous quote:
This is a fairly long writeup of the context, including something on who Almaric was:
And in my AH profile here you can find lots of other stuff on heresy and books to read.
Mark Gregory Pegg's A Most Holy War : The Albigensian Crusade and the Battle for Christendom: The Albigensian Crusade and the Battle for Christendom (Oxford University Press, 2008) is probably the best current work on the Albigensian Crusade. Although with huge reservations about some of his views on what 'Cathars' were, as an overview Walter Wakefield's Heresy, Crusade and Inquisition in Southern France, 1100-1250 is still a great introduction; but I caution that it is filled with real problems (as are most overviews at this point, frankly).
You might be interested in these previous responses by /u/idjet that touch on your question:
I don't really do Albigensian Crusade history so I am of little use, but I know if I were to grab a book on the subject I'd probably default to Jonathan Sumpton's history of the Albigensian Crusade. Sumpton isn't a Crusade Historian but he is currently writing an epic (and very long) history of The Hundred Years War that is by all accounts quite thorough and impressive (I haven't read it yet, each volume is ~500 pages, and it's 3 volumes already while only just passing the midway point of the war). Before his Hundred Years War project began he wrote his history of the Albigensian Crusade and I've heard quite good things about it. I'm not sure if it is the most cutting edge academic history out there (I suspect it's not since it's 15 years old now) but it's an approachable length (270 pages) and can be acquired very cheaply second hand on a well known internet retailer. Definitely a good starting point, worth a look.