The Treasure of Timbuktu by Catherine Palmer

When the Tuareg, a fierce nomadic tribe, come after Tille Thornton, she can't believe they're serious! How can they think she's the legendary Tree-Planting Woman--the one who holds the key to the treasure of Timbuktu? But they are serious. Deadly serious.

Desperate and on the run, Tille finds herself in an uneasy partnership with Graeme McLeod, a daring adventurer who comes out of nowhere to thwart the Tuareg's attempt to kidnap her. Now the two of them are joined against angry tribesmen, crocodiles, a sandstorm, rampaging hippos, and more as they embark on a quest that could bring them the answers they seek--or cost them everything.