Antisuyo: The Search for the Lost Cities of the Amazon by Gene Savoy (1970). Savoy was the American explorer who did more than anyone to bring the Chachapoyas ruins to wider international attention. He located Kuelap and numerous other sites in the late 1960s. This account of his expeditions is dramatic and readable, if occasionally overblown.
Kingdoms of Gold, Kingdoms of Jade: The Americas Before Columbus by Brian Fagan (1991). A solid overview of pre-Columbian civilisations across the Americas, including the Chachapoyas. Useful background reading before visiting any of the northern Peruvian sites.
Lost City of the Incas by Hiram Bingham (1948). Bingham's account of rediscovering Machu Picchu is only tangentially relevant to Kuelap, but it remains the most readable introduction to Andean archaeology and the sense of exploration that still animates the region. Puts the whole enterprise in context.
Peru by the Rough Guide or Lonely Planet (current edition). Both cover Kuelap, Chachapoyas, and the surrounding region in practical detail. The northern highlands section is particularly useful for planning a longer trip that takes in Kuelap, Karajía, Gocta, and Leymebamba.