Derek Walcott, Saint Lucia's Nobel Prize-winning poet, is essential reading before or after visiting the island. His epic poem Omeros (1990) reimagines Homer through the lives of Saint Lucian fishermen and is one of the great works of Caribbean literature. The Bounty (1997) is a later collection, elegiac and rooted in the landscape of his homeland. Walcott grew up in Castries and the island's light, coastlines, and people run through all his work.
For a deeper dive into his writing, Robert D. Hamner's Epic of the Dispossessed: Derek Walcott's Omeros is a solid critical introduction. Walcott died in Saint Lucia in 2017.
Kendel Hippolyte, another important Saint Lucian poet, is less well known outside the Caribbean but worth seeking out. His collection Birthright deals with questions of identity, land, and belonging in the post-colonial Caribbean.
If you want to understand the Kwéyòl language, Jones Mondesir and Lawrence Carrington's Dictionary of Saint Lucian Creole is the scholarly reference, though it's a linguistics text rather than a traveller's phrase book.
For general history and context, Patrick Leigh Fermor's The Traveller's Tree (1950) covers a journey through the Caribbean islands including Saint Lucia. It's dated in some of its perspectives but beautifully written and gives a vivid sense of the region before mass tourism.