The fictionalised life of Syms Covington, assistant to Charles Darwin during his voyage on the Beagle, and clerk for some time after that. McDonald's unusual writing style takes a little getting used to, but results in a very readable -- and, importantly for a historical novel, entirely believable -- story, which I'd happily recommend.
An excellent annotated version of the real Syms Covington's journal is available courtesy of the Australian Science Archives Project.
ISBN 1862300542.