Diary of an Amateur Photographer by Graham Rawle.
It's about a guy who decides he wants to be a photographer. He buys himself an old Rollieflex camera and discovers it still has film in it, which he gets developed. It turns out to be a photo of a murder victim, although he interprets it as a picture of an angel. So he sets out to find out as much as he can about the photograph.
The central character, Michael Whittingham, is one of those socially dysfunctional types who lives entirely inside his own head. People he interacts with always have to rush off to appointments, although he never realises that they just want to get away from him. When he's out and about he counts the women he sees and has sex (imaginary) with every tenth one. His imaginings lead him into quite a bit of trouble, especially when he decides to dabble in nude photography and tries to acquire a model.
As I was reading I was constantly reminded of Mr Bean. This guy is that sort of character.
Everything is recorded in his diary - which is the book. It's basically a record of the events and his bizarre interpretations of them. The book itself is set out as a diary. Whittingham's entries are typewritten (on a typewriter, not a computer) and are interspaced with other bits and pieces he's collected; photographs, newspaper ads, all sorts of odd bits and pieces. It's beautifully put together.
The crime is eventually solved - the answer is tucked away in an envelope glued to the inside back cover.
It didn't take long to read but it's a fascinating study of a man not quite in tune with the reality around him.