The Beatles in India by Paul Saltzman (2000) — Saltzman was a young Canadian filmmaker who spent time at the Maharishi's ashram in 1968 while the Beatles were there. His photographs and memoir give the most intimate account of the two months the band spent in Rishikesh, and capture the ashram life and the river valley around it.
Autobiography of a Yogi by Paramahansa Yogananda (1946) — Not specifically about Rishikesh, but the foundational text of modern Western interest in Indian yoga and spirituality. Many visitors to Rishikesh have read it before arriving; it helps explain the devotional tradition in a way that is accessible without being simplified.
The Himalayan Districts of Kumaon and Garhwal by E.T. Atkinson (1882) — A Victorian-era geographic survey of the region that encompasses Rishikesh and its surrounding hills. Dry in places but invaluable for understanding the settlement patterns, pilgrimage routes, and natural environment of the Garhwal before the railway arrived.