Checked out a number of publishers of the books in the books listed in Africa page of the International Bicycle Fund. Chronicle Books accepts unsolicited submissions - by snail mail. Expect to hear in 3-5 months, they said.
I'm a tech baby and don't print and send anything by snail mail unless I absolutely have to. But I'm sure they are inundated with unsolicited mail and why would they want to spend paper to print something they didn't ask for?