The Ethiopian woman Moses married: And Miriam and Aaron spake against Moses because of the Ethiopian woman whom he had married: for he had married an Ethiopian woman. Numbers 12:1
The Book of Jasher by Rabbi David Rosenfeld
In terms of the reliability of the book, for the most part it is consistent with other similar works. Much of its material appears in the Talmud or in better-known works on the Midrash. It provides detailed accounts of some little-known episodes which are not widely discussed elsewhere, such as Moses’s experiences in the land of Cush (begins in Jasher chapter 73 –(actually begins in Chapter 72 verse 21).
The Ethiopian Woman Moses Married – Moses’s Cushite Wife by Rabbi David Rosenfeld
The story of Moses’s Cushite wife is actually quite cryptic. As you observed, we hear earlier of Moses marrying the daughter of Jethro the Midianite. Yet in the Book of Numbers Moses’s sister Miriam is upset about his having taken a “Cushite” wife. Cush is generally translated as Ethiopia (probably the entire region south of Egypt – see Shemot Rabbah 10:2), a place inhabited by blacks. (See e.g. Jeremiah 13:23: “Can a Cushite change his skin, or a leopard its spots?”) Is this a different wife? And where did she come from?
What the Midrash states
An important introductory point is that the Midrash states that when Moses fled Pharaoh (Exodus 2:15), before arriving in Midian, Moses escaped south to the land of Cush. (Note that Moses was presumably a young man when he fled Egypt, in Midian he married and had two small children, and he was 80 on his return to Egypt at the start of the story of the Exodus. Thus, apparently, many of his early adult years are unrecorded in the Torah.)
Moses first served the king of Cush and then upon his death became king himself, ruling for 40 years. He was given the former king’s widow as a wife but he refused to live with her or worship the Cushite god (Yalkut Shimoni Shemot 168 – https://www.jewishvirtuallibrary.org/yalkut-shimoni)
Based on the Midrash, some explain that the wife under discussion was not Zipporah but the Cushite princess, whom Moses had never lived with (Targum Yonatan, Ibn Ezra alternate explanation, Rashbam, Chizkuni alternate explanation). No doubt unlike his righteous wife Zipporah, the Cushite never embraced the faith and became worthy of joining the nation.
Either way, according to almost all interpretations, the issue was not that Moses took such a wife, but that he had separated from her.
(The Midrash gives different explanations how Miriam happened to find out they weren’t living together.) Many of the commentators who understood that his wife was dark or homely explain that Miriam suspected he separated from her because he found her unattractive. In any event, the Torah records Miriam complaining about something else – claiming that they too are prophets. The implication was that she suspected Moses separated from his wife because he believed a prophet is too holy for married life – as the Midrash puts it, “The elders, fortunate are they but woe to their wives!” She objected to this, but God explained that Moses was an exceptional prophet who had to maintain especially high standards.
The Book of Jasher
The Ethiopian Woman Moses Married Chabod.org — Where Was Moses?
Scripture tells us that Moses left Egypt a young man, and yet he comes back an octogenarian. Where was he for all those years? The Midrash fills in the gap with the following fascinating account:
The Ethiopian Woman Moses Married The Torah: Moses’ Black-Skinned Wife: What Does the Torah Think of Her?
Miriam and Aaron speak negatively about Moses for marrying a Kushite woman. Does their issue have to do with her skin color? Miriam’s punishment may hold the key.
The Ethiopian Woman Moses Married The Torah: Moses and the Kushite Woman: Classic Interpretations and Philo’s Allegory
Ancient interpreters debated the identity of Moses’ Kushite wife and the nature of Miriam and Aaron’s complaint.