The Sisters Brothers by Patrick DeWitt

Eli and Charlie Sisters are famous gun slinging killers for hire in the late 1800's.  They are employed by the Commodore, a powerful nasty figure who operates out of Oregon City in the Oregon Territory.  They are on their way from Oregon City to the gold fields of California to kill Hermann Kermit Warm and steal his formula for an amazing liquid which makes the hard to see gold shine in the river beds.  Both brothers seem to kill without a bit of remorse and the killings are frequent which makes the fact that this book is full of humor hard to explain.  The language and voice are pure Twain.  The ending is both strange and satisfying which is good because somehow DeWitt has created two killers you kinda like.