If you're looking for actual myths rather than simply a broad overview of multiple religions there's a few great texts out there. I'm assuming she is African-American in which case it would be logical to focus on West and Central Africa.

Mandinka and Bambara

Essai sur la religion Bambara. Germaine Dieterlen. Presses universitaires de France, 1951

Yves Bonnefoy (dir.), Dictionary of mythology and religions of traditional societies and the ancient world , Paris, Flammarion, 2 volumes, 1999 ( 1st edition 1981).

Youssouf Tata Cissé , Wa Kamissoko , Soundjata, the glory of Mali (The Great Gesture of Mali, volume 2) , Paris, Karthala, "Man and Society: History and Geography", 1991, 2nd edition 2009

Youssouf Tata Cissé , Wa Kamissoko , The Great Gesture of Mali. From the origins to the foundation of the Empire , Paris, Karthala, 1988, 2nd edition 2007.

Dieterlen, G. (1957). The Mande Creation Myth. Africa, 27(2), 124-138.

Leo Frobenius, Douglas C. Fox. African Genesis: Folk Tales and Myths of Africa, 1937, New York: Stackpole Sons.

Cissé, Youssouf. La Confrérie des Chasseurs Malinke et Bambara: Mythes, Rites et Recits Initiatiques. Paris, 1994.

The Bambara, "Iconography of Religions," sec. 7, fasc. 2 (Leiden, 1974)

Abdoulaye Sako. Stephen Bulman. Valentin Vydrine. The Epic of Sumanguru Kante. 1997.

Cisse, Youssouf. Le sacrifice chez les Bambara et les Malinké. Systèmes de pensée en Afrique noire, Cahier 5. 1981.

Pascal James Imperato. Buffoons, Queens, and Wooden Horsemen
The Dyo and Gouan Societies of the Bambara of Mali. Kilima House. 1983.

Dogon

Marcel Griaule. Conversations With Ogotemmeli: An Introduction to Dogon Religious Ideas – January 1, 1970

Fulani

Amadou Hampaté Bâ. Njeddo Dewal, mère de la calamité: Conte initiatique peul. Nouvelles Editions africaines. 1985.

Amadou Hampaté Bâ. Kaïdara, récit initiatique peul. Three Continents Press, 1988.

Amadou Hampaté Bâ. Koumen: texte initiatique des pasteurs Peul. 1961.

Amadou Hampaté Bâ. L'Éclat de la grande étoile (1974)

Songhai

Rouch, Jean. La religion et la magie Songhay. Paris: Presses universitaires de France. 1960.

Dupuis Yakouba, M. A.: Les Gow ou Chasseurs du Niger, Legendes Songaï de la Région de Tomboctou (Paris: Leroux, 1911)

Akan

Akan-Ashanti Folk-Tales. Collected and translated by ... R. S. Rattray ... and illustrated by Africans of the Gold Coast Colony, 1930.

Beckwith, Martha Warren, and Helen H. Roberts. Jamaica Anansi Stories. New York: American Folk-Lore Society, G. E. Stechert & Co. Agents(1924).

Appiah, Peggy. The Pineapple Child and other tales from Ashanti. London: Evans, 1969.

Appiah, Peggy. The Children of Ananse. London: Evans, 1968.

Appiah, Peggy. Ananse the Spider: Tales from an Ashanti village. New York: Pantheon, 1966.

Dahomey

Melville J. Herskovits, Frances S. Herskovits. Dahomean Narrative
A Cross-cultural Analysis. Northwestern University Press. 1958.

Yoruba

Ulli Beier. Yoruba Myths. Cambridge University Press. 1980.

Alfred Burdon Ellis . The Yoruba-speaking peoples of the Slave Coast of West Africa; their religion, manners, customs, laws, language, etc. 1894.

Harold Courlander, Larry Lurin. Tales of Yoruba Gods and Heroes. Crown Publishers. 1973.

Ijaw

Clark-Bekederemo J. P. (trans.) The Ozidi Saga. Howard University Press 1991

Ulli Beier. African Creation Myths. Pearson Education. 1966.

Duala

Ralph A. Austen. The Elusive Epic : the Narrative of Jeki la Njambe in the Historical Culture of the Cameroon Coast. African Studies Assn. 1995.

Bassa

Pierre Ngijol Ngijol. Les Merveilles Africaines: Les Fils de Hitong . Centre d'Edition et de Production pour l'Enseignement et la Recherche Yacoundé. 1985.

Fang

Nguema, Zwe. Un Mvet de Zwe Nguema, chant épique fang. Les Classiques Africains. 1972.

Ndoutoume, Tsira Ndong

Le Mvett I Présence Africaine. 1970.

Le Mvett II Présence Africaine. 1972.

Le Mvett: L'homme, la mort et l'immortalité. Présence Africaine. 1993.

Belinga, Eno. L'Épopée camerounaise, mvet: Moneblum ou, L'homme bleu. Yaoundé : Centre d'édition et de production pour l'enseignement et la recherche. 1978.

Ndoutoume, Daniel Assoumou. Du Mvett : Essai sur la dynastie Ekang Nna. Paris: The Harmattan. 1986.

Minlan Mi Mved (chants lyriques) recuellis par l'Abbé Tobie Tsala Atangana , Recherches et études camerounaises (Yaoundé). 1960/1961.

Mevoula Olinga. La guerre des Mekemeze, précédé de, Aneng-Bikale, l'esclave-au-cœur-de-panthère. Editions CLE, 1980.

Awona, A. 'La guerre d'Akoma Mba contre Abo Mama (épopée du mvet)', Abbia 9/10, 1965

Mbundu

Chatelain, Héli. Folk-tales of Angola: Fifty tales, with Ki-mbundu text, literal English translation, introduction, and notes. New York, Negro Universities Press [1969, ©1894]

Welcome to the deep and fantastical worlds of African mythology.

One factor here is also the incredible diversity in Africa. Remember, our species had about 200.000 years of head start building up diversity of language, genetics, culture and history in Africa compared to any other continent. So knowing she is from West Africa in a way narrows it down less than for someone just knowing her family is from "Eurasia" once past the shared experiences of abduction and slavery uniting the majority of African-Americans (assuming you are Americans.)

Literally. I know that can be hard to wrap one's mind around if you have grown up with the "race" concept. But on average, there is arguably is less genetic, historical and linguistic difference between my pasty Norwegian ass and a random person from Mongolia than between a Baka and a Bantu from West Africa. (As an evolutionary biologist working in Africa that point is pretty close to my heart.)

Of course, that does not stop her or anyone from learning as much as possible about all the different cultures and their myths and histories. Do you live in the US, or somewhere else? I.e. is it difficult for you to go to, say Senegal, Gambia or another of the safe countries to visit? A lot of myths are easiest to come across talking to people, picking up little books of local myths that have been gathered by local enthusiasts but never published on an international market and so on.

That said, if I was to suggest one book to start with, I guess you could do a lot worse than starting by reading up on history and legends from Mali. And if so, Sundiata by Mamadou Kouyaté is a good place to start.

While waiting for some insightful answers, I'd like to say I always found Oxford University Press' "Very Short Introduction" series as a good kickstart for most fields. Even when the book falls short, their bibliography often points to the right direction.

It appears there is a Very Short Introduction for African Religions: https://www.veryshortintroductions.com/view/10.1093/actrade/9780199790586.001.0001/actrade-9780199790586?rskey=EbSk38&result=3

And another for general history:

https://www.veryshortintroductions.com/view/10.1093/actrade/9780192802484.001.0001/actrade-9780192802484?rskey=EbSk38&result=1

Not quite what you are asking for, but if your partner is a black American I might have some sources that you would be interested in. I once did a presentation on how African (most often West African) religion and magic came into the United States through African Slaves and how it became layered with Christianity. Some sources I came across when doing research may be of interest to you or your partner

Some sources that I liked were:

Old Ship of Zion: The Afro-Baptist Ritual in the African Diaspora by Walter F. Pitts

Slave Religion: The "Invisible Institution" in the Antebellum by Albert J. Raboteau

Black Magic: Religion and the African American Conjuring Tradition by Yvonne P. Chireau

African American Religious History History: A Documentary Witness edited by Milton C. Sernett

Africans in Colonial Louisiana: The Development of Afro-Creole Culture in the Eighteenth-Century by Gwendolyn Midlo Hall.

I'd say that the first three are the better three for your purposes though and the last two are less focused on what you want, but will have some additional information.

The Teaching Company has an excellent course called "Great Mythologies of the World" with 8 30 minute lectures on African Mythology by Dr. Julius H. Bailey (University of Redlands) that are a great starting point. While African Mythology definitely gets the shorter end of the stick, Donna Rosenberg's "World Mythology" still has about 75 pages giving an overview of African mythology with 5 or 10 representative myths, and the introductory material is a great primer on the study of mythology as a whole.

Also on Audible, there’s a series called Great Courses which is just academics lecturing about their field. There’s a course called ‘great mythologies of the world’ which is like a primer on world mythology. There’s 12 thirty minute lectures on Africa which is probably a lovely place to dip your toes, especially as you can then compare it to Europe, Asia etc (If you want)