Bill, the Galactic Hero by Harry Harrison

It was the highest honour to defend the Empire against the dreaded Chingers, an enemy race of seven-foot-tall lizards. But Bill, a Technical Fertilizer Operator from a planet of farmers, wasn't interested in honour - he was only interested in two things: his chosen career, and the shapely curves of Inga-Maria Calyphigia. Then a recruiting robot shanghaied him with knockout drops, and he came to in deep space, aboard the Empire warship Christine Keeler. And from there, things got even worse...
Binding: Paperback / softback