Ramuntcho by Pierre Loti (1897). Loti's novel set in the Basque Country and Landes borderlands follows a young man caught between love, the church, and the pull of the mountains. Loti grew up near the Landes coast and writes the landscape — pine forests, Atlantic light, the flat country stretching away from the Pyrenees — with the authority of someone who knew it in his bones. It is sentimental in places, but it captures the world that Mont-de-Marsan sits at the edge of better than any travel account.
Les Landes: Une forêt et des hommes by Jean-Pierre Larrieu. A detailed account of how the Landes pine forest came to exist — the nineteenth-century project that drained the marshes, planted millions of trees, and transformed one of the most inhospitable landscapes in France into a productive forestry region. The book explains why the countryside around Mont-de-Marsan looks the way it does and what it cost the people who were already living here. Difficult to find in English, but the French is accessible.
Eleanor of Aquitaine: A Life by Alison Weir (1999). Eleanor ruled the Duchy of Aquitaine — the region that contains the Landes — in the twelfth century, and her story encompasses the courts of both France and England, two marriages to kings, and a political life of extraordinary range and tenacity. Weir's biography is the most readable English-language account. It sets Mont-de-Marsan in its deeper historical context: this was Gascon country, part of the great southern duchy that resisted northern French power for centuries.