You don't mention what level of study this is, so apologies if this is pitched wrong. I'd also welcome anyone else who knows of materials to chime in.

One thing to recognize is that the British in (then) Southern Nigeria generally used "Ibo" and not "Igbo." N. Whitridge Thomas's multi-volume 1913-1914 ethnographic study of Igbo society under colonial rule (Anthropological report on the Ibo-speaking peoples of Nigeria, which I think are all on Google Books--search "Ibo Nigeria" and they pop right up) is one of the very first such studies, followed in 1921 by Basden's; Thomas expresses the banally scientific, paternal imperial mindset pretty effectively. You can't take him as objective about his subject, but he will give you a contemporary look at the colonizing mind in the era just before Chinua Achebe's second novel in the famous triptych (Arrow of God). So for western perspectives and influences, at least scientifically, you might consider Thomas; journals like those of the Royal Society, Royal Anthropological Society, and Royal Geographical Society (among others) might also have essays on Igbo society--again, under Ibo almost always. If you go a little further to the coast and into Cross River you can talk about Efik and Ibibo groups (among others); Mary Slessor ran a mission there and shaped some of the discourse on those Igbo-adjacent societies, and there are sources out there that should be accessible. Mary Kingsley also spent a bit of time with her, and so her accounts (e.g., Travels in West Africa) had a wide readership too but she journeyed much more widely.

It's possible that a journal of one of the learned societies would give you a more manageable piece to unpack. German and French societies have a few 19th-century essays. But usually the discussion of specific groups of people was not something attractive to European mass media unless it involved a an identifiable foe or hierarchy (e.g., the Zulu War of 1879).

(Addendum, although I know you're not looking for these: For actual Igbo sources, however, you'd have a harder time--what we do have tends to be secondhand, or oral history that's mostly found in Nigerian historical publications. Those are not widespread outside Nigeria as they should be, which is a cause for great regret. If you do need that, you might look at the sources cited by historians who work on that area, like Judi Byfield, Carolyn A. Brown, or Nwando Achebe (yes, she's Chinua Achebe's daughter), although many of theirs are direct oral history.)

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