Hard to pinpoint exactly why this didn’t work for me, but a large part of it was the main character. He describes himself as average. He is very average, though he tries to make himself seem more interesting than he is. I just didn’t particularly care about him or what had happened to him. 
When the book opens our adult narrator is remembering his teenage years and the friendships he forged at that time. He sets himself up as someone to trust, someone who we can believe as he recalls his early years. Unfortunately, memory is a funny thing and what one person recalls may not be exactly what another does. And that blurring of recall definitely seems to be a feature in this story.
An ordinary man, divorced, is bequeathed a diary from an old school friend and it prompts him to try to make sense of the events that took place around the time his friend committed suicide. This is done through incredibly boring email exchanges with his ex-girlfriend who got together with his friend after they split up. His ex-wife questions why he feels the need to look back at events so far in the past, and we never really get a satisfactory answer. Perhaps there isn’t one.
There is a bit of a twist which he/we come to see at the end, but it all felt a little too late. Vivian, his ex, says he never got it and probably never would. I think that just about sums up my reading experience!