West Africa generally comprises the area made up of the following countries: Benin, Burkina Faso, Cameroon, Cabo Verde, Chad, Côte d’Ivoire, Equatorial Guinea, The Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, and Togo.
The Library of Congress Subject Heading system only uses "Christianity -- [Country name]" or "Christianity -- Africa". It's very unlikely to see "Christianity -- West Africa" as a geological subheading.
Likewise, it's important to note that searching "Christianity -- Africa" will not include books whose catalog record only includes "Christianity -- [Country name]" as a subject heading -- even if that country falls within Africa.
Spirit, Structure, and Flesh : Gender and Power in Yoruba African Instituted Churches by 
Pentecostalism, Globalisation, and Islam in Northern Cameroon : Megachurches in the Making? by 
Faith and Charity: Religion and Humanitarian Assistance in West Africa by 
Faith, Power and Family : Christianity and Social Change in French Cameroon by 
Creativity and Change in Nigerian Christianity by 
Christian Churches in Dahomey-Benin : A Study of Their Socio-Political Role by 
Faith in Empire : Religion, Politics, and Colonial Rule in French Senegal, 1880-1940 by 
A heritage of faith : a history of Christianity in Nigeria by 
Alexander Crummell (Eighteen Nineteen to Eighteen Ninety-Eight) and the Creation of an African-American Church in Liberia by 
Of God and Maxim Guns : Presbyterianism in Nigeria, 1846-1966 by 
"White" Americans in "Black " Africa : black and white American Methodist missionaries in Liberia, 1820-1875 by 
Communication and Conversion in Northern Cameroon : The Dii People and Norwegian Missionaries, 1934-1960 by 
The Residue of the Western Missionary in the Southern Cameroons : : The Christian Village: a Sad Tale of Strife and Dissension by 
A History of Anglican / Methodist Collaboration in Nigeria Within the Yoruba Socio-Cultural Context by 
Igbo Culture and the Christian Missions 1857-1957 : conversion in theory and practice by 