Good reading on Niger is scarce. Here are a few titles worth tracking down before you go.
Riding the Demon: On the Road in West Africa by Peter Chilson is probably the best travel writing on Niger specifically. Chilson spent years in the country as a journalist and captures both the texture of daily life and the brutal logic of bush taxi travel. Honest and unsentimental.
To Timbuktu by Mark Jenkins covers a canoe expedition down the Niger River from its source, touching on the cultures and landscapes of the Sahel. Good context for the river's role in shaping West African civilization, though Niger (the country) gets less attention than Mali.
The Cinematic Griot: The Ethnography of Jean Rouch by Paul Stoller examines the life and work of the French filmmaker who documented Nigerien life from the 1940s through the 1990s. Rouch's films — many shot in Niamey and the Niger River valley — are an extraordinary archive of a world now largely gone.
Marriage in Maradi: Gender and Culture in a Hausa Society in Niger, 1900–1989 by Barbara M. Cooper is an academic work but accessible enough for serious travelers. It gives real depth to the Hausa south and challenges easy assumptions about Islam and women's agency in the region.
For practical planning, the Lonely Planet West Africa guide covers Niger in the broader regional context. Dedicated Niger guidebooks are essentially nonexistent in English.