I asked this question six months ago and got no responses, so I'm asking again because I'm very interested in knowing if such stories are biased hyperbole or a legitimate description of the scale of destruction that occurred during those conquests.
I don't have the book with me so I cannot give an exact reference for where I've heard of these pyramids of skulls, but I believe I read it in Rene Grousset's Empire of the Steppes: A History of Central Asia, published in 1970, ISBN: 978-0813513041.
As a followup, I have heard this associated with Nader Shah as well - any basis for that, or part of his mythology as the "last great conqueror" of Asia?