Crooked Little Vein by Warren Ellis.  

Comics fans will be familiar with Ellis' irreverent and often filthy sense of humor. Crooked Little Vein is a quicker textual dose of the same, and despite the lack of images, no less graphic. A down and out detective with $3 and change to his name is one afternoon approached by powerful government figures and charged with recovering the Secret Constitution of the United States. (This is a document bound in the skin of an alien beaten to death by Benjamin Franklin, and traded by Nixon for the services of an asian prostitute.) Given the proverbial carte blanche, Mike hooks up with a polyamorous indie chick doing her thesis on the sexual underworld, and begins his journey across the US in search of the current owner of the book. At it's heart, this is a dirty road trip novel as only Ellis could write it – the outsider's perspective on the sometimes bizarre mix of morality and filth that the American landscape provides. The only drawback to this is that Ellis does seem to write analogs of himself fairly often, and Mike is no exception; this would be a problem if they weren't so damned funny.