Lords of the Atlas by Gavin Maxwell (1966) is one of the great books about Morocco — the story of the Glaoui clan, the Berber warlords who ruled the south from their Kasbah in Marrakesh and maintained their power by collaborating with the French, until independence swept them away. It reads like a novel but is history. The Souss valley around Agadir was part of the territory they controlled.
Elias Canetti's The Voices of Marrakesh (1967) is a slim travel memoir from a Nobel laureate — perceptive, strange, and nothing like most travel writing. Canetti spent several weeks in Morocco and the book captures something about the sensory experience of the country that guidebooks miss.
Morocco: The Traveller's Companion edited by Margaret and Robin Bidwell collects accounts of Morocco by European travellers over several centuries, including sections on the Souss and the south. Useful for understanding how the country looked before mass tourism arrived.