Just after I wrote the last review, I looked on Patrick Ness' website and found the name of the last, final book in the series for anyone that wanted to know.
On Page 430 of The Knife of Never Letting Go, Todd has a flash of memory: “War makes monsters of men, I hear Ben saying.”
On Page 102 of The Ask and the Answer, Viola remembers it, too: “War makes monsters of men, I say, quoting Ben...”
The name is.....
Monsters of Men
Out 3 May in the UK, Ireland and Australia, 12 May in New Zealand, 28 September in the US
Hope that helps anybody. Oh my, it's out in only 15 days :)