How historically accurate is the book “Thug or a million murders” by James L. Sleeman about Thugs' suppression made by William Henry Sleeman?

by Q_uoll

I am currently reading this book and I'm wondering if it is historically correct, is completely inaccurate, or contains any truth.
I'm particularly interested about Thugs' origin and the claim that the “sculptures in the Caves of Ellora” (the author says they were created in the eighth century) depict Thugs' operations.
Did Thugs really existed? Did they operated in the way described in the book?

GreatStoneSkull