Lonely Planet and Rough Guide for The Gambia are the best.  Order at any good bookshop or on line through Amazon.

BBC news website has a country profile of The Gambia, which is kept up to date.

A great detailed map of The Gambia could be bought at Kombo Beach Hotel, Kololi.  Its a German one!

If you have an e-book reader you might find the 'Malinding' series of e-books from Amazon helpful. They are novels set in The Gambia and include a lot of information about village life and customs. Cost about £2 each !