To Timbuktu by Mark Jenkins. A vivid account of a Niger River expedition that passes through Guinea and captures the spirit of West African travel -- the breakdowns, the hospitality, the absurdity, and the beauty.
The Fortunes of Africa by Martin Meredith. A sweeping history of the continent that gives essential context for understanding Guinea's place in West Africa, from the Mali Empire through colonialism to independence.
African Guitar: Solo Fingerstyle Guitar Music from the Western Sahel -- not a book but a companion resource for understanding Guinea's extraordinary musical heritage. The country produced some of Africa's greatest musicians, and reading about the djembe and balafon traditions before visiting enriches the experience enormously.
Camara Laye's The Dark Child (L'Enfant Noir). A memoir of growing up in colonial Guinea, widely considered one of the finest works of African literature. Laye's descriptions of Malinke village life around Kankan remain vivid and moving.
Lonely Planet West Africa covers Guinea in a practical chapter that is one of the few up-to-date English-language guides available. Essential if you are traveling independently.