There are no credible historical books on pre-Islamic Somalis since there was no written tradition among pastoralists in those days. What you can find is information indirectly, by reading about ancient herders and Cushitic groups from neighbouring countries, it's interdisciplinary, but you'll learn a great deal as they varied greatly in culture from even 5000 years ago.
For example, there is much information from the archaeological findings of the first Pastoral Neolithic who were classified in the same cluster of peoples the ancestors of Somalis belonged to. Who they were and how they lived is reasonably documented, also a bunch of new information has been added from recent genetic research, which strengthens previous anthropological works.
Soon we can expect researches to excavate and uncover a lot of ancient history in the homeland as security and extant human conditions and relations improve because I've seen random ancient artefacts pop up on the internet from there and it's well-known looters sell it to the black market. And we know there exist ancient sites, though not protected, and I'm certain there are many more.
I'm interested in how this fits into the broader Cushitic migration narrative and the cultural and religious diversity that existed after we arrived – the social dynamics between local foragers and proto-Somalis – their settlement patterns and the use of structural and functional properties compared to other related groups respectively.
In conclusion, maybe it's a good thing that we don't know specifically much about our early history as it consequently challenges assumptions and leads to more research with less bias and much mystery and excitement.
