The Wonder That Was India by A.L. Basham (1954) remains the single best introduction to the civilisation that built Kanchipuram's temples. Basham covers the full sweep of Indian history, religion, philosophy, art, and literature up to the arrival of Islam — the Pallava period and its art-historical significance are given proper weight. Dense but readable, and the most useful book you can take to any major archaeological site in south India.
The Pallavas by K.R. Srinivasan (Archaeological Survey of India, 1964) is the scholarly authority on the temples of Kanchipuram. Srinivasan catalogues each monument with plans, photographs, and inscriptions, placing them within the development of Dravidian architecture from the Mahendravarman period through the later Pallavas. It's a reference work rather than a narrative, but invaluable if you want to understand what you're looking at.
South India: A Guide to Tamilnadu, Kerala, Goa and Karnataka by various editions from Footprint or Lonely Planet — not literature, but the best practical context for understanding Kanchipuram in relation to the wider circuit of south Indian temples (Mahabalipuram, Chidambaram, Thanjavur, Madurai). Reading it before you travel helps you prioritise.
A Passage to India by E.M. Forster (1924) is not set in Tamil Nadu, but its central preoccupation — the encounter between the rationalist European mind and the irrational, overwhelming presence of Indian sacred space — is the most useful literary preparation for entering the active pilgrimage temples of Kanchipuram. The Marabar Caves chapter, in particular, is the best fiction ever written about what Hindu temples can do to a Western visitor.