This quirky story of difficulty in dealing with the end of a relationship reminded me of several young adult books. I suppose it's just the age of the protagonist that makes it an adult book. Disconsolate Sid Higgins is an endearing dysfunctional character we want to root for, and the mystery of the postcards will make for cult discussion groups. Good fun.
Similar YA books:
Someday This Pain Will be Useful to You - character’s troubled soul
Marcelo in the Real World – protagonist’s inability to deal with the world
Perks of Being a Wallflower – is much darker, but also has dysfunctional (grieving?) protagonist
The Flawless Skin of Ugly People – written communication with a distant loved one
Maynard & Jennica – quirky tone, information parsed out to reveal story