The Friendly Islands Bookshop near Fund Management House on Taufa'ahau Road sells books by local authors 'Epeli Hau'ofa, Pesi Fonua, Konai Helu Thaman, and Tupou Posesi Fanua. They also carry Tongan music on compact disc. Friends Tourist Center has a used book exchange. You must bring along a book to trade and pay a small fee.
The Vava'u Press, corner Railway Road and Hala Holomui, sells some excellent books on Tonga, plus back issues of their magazine Matangi Tonga which switched to an internet-only edition in 2003.
There's a good library (weekdays and Saturday mornings) in the University of the South Pacific, Tonga Campus near the Golf Club at Ha'ateiho, out on the road to the airport. Ask to visit the Pacific Collection. The Veitongo bus stops here.
A few topographical maps are available at the Lands and Survey Department in the back yard of the Ministry Block on Vuna Road on the waterfront.
Navigational charts of Tonga, Samoa, and New Zealand are available at the Hydrographic Office at Touliki Naval Base, just east of Queen Salote Wharf. The hassle here is that you must first go the base to select your charts and get an invoice, then you must visit military headquarters near Centenary Church in town to pay, then return to the base with the stamped invoice to pick up the charts.