Online is rough, since most serious studies of of pre-colonial African history have been written in the last half-century or so, and so is much of it is still behind copyright walls. Even so, West Africa is still huge, and you might want to narrow that further.
At the accessible level (although not sure if online) practically anything by Basil Davidson is certainly worth a visit. But the man has a definite perspective.
(And then you have guys like Cheikh Anta Diop, who do quite good pre-colonial history, but have really big chips on their shoulder and dubious political agendas that distort their usefulness and turn them into borderline cranks.)
The UNESCO-sponsored seven-volume general history of Africa, although very broad-sweeping and not always so detailed, are generally well-written and very useful for overviews. Happily, all the volumes are available
online at UNESCO.
Along the same general lines is Roland Oliver's "Medieval Africa, 1250-1800". Not online as far as I know. Dunno if it is any good - I haven't read it yet. But if you come across it, let me know how it is.
Unfortunately, pre-colonial African history tends to have few written sources. The Arabs and early Portuguese are the closest you can get. Otherwise, you'd have to rely on archaeology and oral histories. Even so, it is still relatively under-researched - I can name a dozen or so relevant pre-colonial accounts that have never been translated into English or properly digested by scholars yet (if you want a project....).
If you are an insomniac or a masochist, take a peek at the "The Negroland of the Arabs" by W.D. Cooley (available online
here or
here). It is the first valiant (if tedious) attempt to re-construct the history of Medieval West Africa from contemporary Arab sources. Although horribly outdated - 1841! - it is still strangely profitable. But beware - 1841 is before most inland explorations and before archaeology. So "Negroland" is really just pure book-to-book knowledge (on the upside, despite its title, it is also mercifully bereft of the racism and cultural prejudices that would mar later academic histories). If you prefer your Arab histories unamalgamated and unmessed-around-with, then you need Levtzion & Hopkins's "Corpus of Early Arabic Sources for West African History", the only English translations available for many of them (alas, not online; but if you can read French, there are available online French translations of most of these same Arab chronicles I can point you to).
For eyewitnesses, who actually saw and described pre-colonial West Africa, Leo Africanus is, of course, still the earliest go-to man. He traveled through West African Sahelian kingdoms in early 16th C. and went on to write a travelogue c.1526. It was first published in Italian in Ramusio's 1550 collection (if you want to read the original Italian version:
Descrizione dell' Africa). Happily, an English translation (if still archaic) of Leo Africanus is available online as "The History and Description of Africa"
vol. 1,
vol.2 ,
vol.3
If you can read Portuguese, there are so many untranslated accounts I could throw at you. They did not really go much inland, but the coastal kindoms of West and East Africa are usually very well described. One i tried a hand at translating (gave up after a few pages) was the account of the Cape Verdian officer Andre Alvares d'Almada who gives a damned detailed description of the Senegambian kingdoms in his 1594 "Tratado breve dos Rios de Guiné do Cabo-Verde"
online. Alas, he doesn't go further inland. Still, it contains some of the best descriptions of pre-colonial Senegalese politics, society and cities I have come across. Including the earliest (if charmingly weird) description of "griots" I know.
Later 18th/19th C. European travelogues can be found by the gazillion - Portuguese, English, French, German whatever. Many are into previously unvisited areas, so the tinge of European colonialism had not yet distorted them completely (although trade - esp. the slave trade - did stretch into the deep interior, so nothing was really intact anymore.)
Dunno if any of this is useful to you. But there it is. If you give me more precise details of what you're honing in on, maybe I can provide more useful assistance.
