The intro to the book begins with the sentence:
"Mahatma Gandhi and Jawaharlal Nehru helped create the myth of a nonviolent ancient India while building a modern independence movement on the principle of nonviolence (ahimsa)."
Even if the statement that these two leaders created the myth of non-violent ancient India can be substantiated through proper documented of where and how they created this myth, I can say with certainty that ancient India was never imagined during or later to the independence movement, either by the educated Indians or by the common public as an India without wars or without harsh punishments or without harsh ways of crime control etc. Wars etc. of ancient India were common place from the lowest level history text books, or the most common sense /layman's /non-professional books on history to the most professional university level books on history. Nationalist historiographers glorified the valour of the ancient Indian kings in their history writings. The legends about sahivaji contain descriptions of Jijiabai teaching young Shivaji , the stories of Ramayana and Mahabharata as stories of valour (whereas Anandavardhana looks at the angirasa of these books as S'aanta Rasa and some other authors of poetics, plays or poetry view Ramayana as a book of Karuna Rasa, my teacher views both these books as books of Dharma veera). Even the most ardent followers of Gandhi compared him to Buddha , Buddha now newly known as an epitome of non-violence through the modern history works unlike in the pre-modern Indian literature where he was either an avatar of Vishnu or a vaada poorvapakshin for the Vaidika schools of philosophy. The comparison of Gandhi selectively to Buddha was required only because the remaining picture of ancient India was dominated by the incidence of wars etc.
I have to read Sri Upinder Singh's book to be able to say if the book is really an attempt to demolish a non-existent impression of India in the general Indian people.