Sorry for the delay in getting to your question.

I had a hard time thinking of a book to recommend, because the Bantu expansion is not a topic many authors try to tackle. Since the migrations happened in a pre-literate era, our knowledge comes from the disciplines of Archaeology and Linguistics. Archaeology works tend to discuss the appearance of iron working and modes of living that are "characteristic" of Bantu-speaking societies. Liguistic studies focus on similarities in vocabulary and grammar to define "language clusters" and thereby create a narrative of expansion.

In short, the literature of both of these disciplines can be somewhat dry.

Fortunately, I can recommend the book Why People Move: Migration in African History by Phillip Curtin. I like this book because it can distill the conclusions that Archaeologists and Linguists make into language understandable by a layman. The only caveat is that Why People Move has a wider scope than only the Bantu expansions, it also deals with Nilotic migrations, migrations in the Sahel, and the African Diaspora created by the slave trades.

Know any good books about it or African empire growth in general?

For growth of African empire in general, I would start with Medieval Africa by Roland Oliver and Anthony Atmore. It covers the era from 1200-1800, with a scope covering the entire continent. Certainly, you will miss a fair amount about earlier civilizations like Kush/Nubia, Aksum, ancient Ghana, and early stages of Swahili civilization. But, for the era that it does cover, it does a creditable job of hitting the highlights in a single book.