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. Search for this book on Wikipedia or isbn.nu.