How To Be Bad (St. Martins Griffin, 2005), by David Bowker.
Mark Madden, a mild-mannered young bookshop owner is having a bad day. A stranger, with an amazing resemblance to Jesus, has come into the shop and set fire to one of his more expensive books. Mark has also been beaten up by a tattooed neanderthal and had his bike stolen. Oh, and he has inadvertently murdered a pigeon by running it over with the aforesaid bike.
At the Casualty Department, he meets an ex-girlfriend who he last saw giving his art teacher a blow job at Mark’s 18th birthday party. When they were still going out. But he’s a forgiving sort of chap (not to mention besotted) and they start seeing each other again. It all goes downhill from there. Caro Sewell demands a lot from her boyfriends, including murder.
This is pitch-black humour with a consistent, deadpan tone. Watch Mark learn to be bad.