On Your Own in El Salvador by Jeff Brauer, Julian Smith, and Veronica Wiles — One of the few dedicated English-language guidebooks specifically covering El Salvador. Practical and detailed, though now dated.
Adventuring in Central America: Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica, Panama by David Rains Wallace — Good background on the region's natural history and national parks.
Bitter Grounds by Sandra Benítez — A novel spanning several generations of a Salvadoran family, offering a vivid and painful portrait of the country's social history and the roots of the civil war.
The Massacre at El Mozote: A Parable of the Cold War by Mark Danner — The definitive account of the 1981 massacre in which US-trained Salvadoran soldiers killed nearly a thousand civilians in the village of El Mozote. Essential background reading for understanding the civil war and its legacy.
Long Night of White Chickens by Francisco Goldman — A novel partly set in El Salvador that explores the violence and disappearances of the 1980s with literary depth.