Trimingham's A History of Islam in West Africa is quite dated. For instance, it relies on problematic older assumptions typical of the scholarship of its era. For example, the Almoravids are assumed without question to have sacked or defeated ancient Ghana. In addition, the author relies on problematic assumptions of "Hamitic" races and "Negro" (while also considering all Berbers "white") while also describing colonialism as largely beneficial. Indeed, based on the rise of theocratic Islamic states in the 19th century and the endless wars and slave-raiding created by these theocratic states and the military adventurers like al-Hajj Umar, Trimingham appears to see in European colonialism as saving West Africa from endless wars.
Nontheless, Trimingham's work provides an excellent synthesis of the historical knowledge of its era. One sees exactly what the dominant trends were based on oral history and internal and external textual sources to reconstruct the origins of trans-Saharan trade, the spread of Islam, and the various polities and kingdoms of the "Sudanese" style that developed in the western and central Sudan, from Senegal to Chad. Despise the author's misgivings about the reliability of the oral sources for earlier moments in the development of states like Takrur, Ghana, and Songhay/Kawkaw, they add a wide array of new details to the cultural and historical development of the various peoples of the region.
Surprisingly, we learn less about Islam in this region than the history of kingdoms, traders, and trading centers like Jenne or Timbuktu, but Trimingham rightly sees Islam in pre-19th century West Africa as mainly limited to towns, traders, and kings who professed it but did not impose it on their subjects (although this is contradicted by states like Takrur, whose first king, War Jabi, did compel his subjects to Islam). Overall, Trimingham's study is more about political and social history with appropriate emphasis on Islam for certain key moments, developments, or changes in religious practice, such as the rise of Sufism, jihadist 19th century movements, and colonialism aiding Islam's expansion amongst "pagan" peoples. Sometimes one thinks Trimingham sees in West African "traditional" religion something anti-universalist (unlike Islam) and closed off to the wider world, so perhaps more recent scholarship has approached the delicate question of Islam and "traditional" religion more convincingly.