Guards Guards by Sir Terry Pratchett

Defending Ank-Morpork against this threat is the entire, underpaid, undervalued City Night Watch - a drunken and world-weary Captain, a cowardly and overweight Sergeant, a small opportunistic Corporal of dubious parentage...and their newest recruit, Lance Constable Carrot, who is upright, literal, law-abiding and keen. Aiding them in their fight for truth, justice and the Ankh-Morporkian way are a small swamp dragon and the Librarian of Unseen University (who just happens to be an orang-utan).