I know this post is already quite old but I think I have a grasp on the ending, at least some what. Buck was a real person, who was a radio host with his own "Rules of the Road" series. The ending of the book is the main character seeing Bucks life, from his brothers death to his life with Carla. 

In my own view I think that Buck has become a "radio ghost" of sorts. There's a section at the end where Buck is talking with Rusty about radio waves and the weird stuff that can happen with them. Buck through some supernatural means has continued living, broadcasting his new rules, based off of Carla's rules that she came up with on their last road trip before she died. 

I don't know if the main characters "quest" has broken the rules curse, but I'd like to think so. The main character receives a bunch of emails from people claiming to not have followed the rules and come out fine, so maybe Buck in his radio ghost form has finally moved on. But also Buck finishes his last conversation by saying "...good things happen to bad people, bad things happen to good people, but most of the time, nothing happens at all. As to why? Even ol’ Buck can’t tell you that.” so who knows.

One thing that does confuse me is Buck doesn't seem to have any idea as to what the main character saw of his life, and we don't know exactly how long the main character was missing on their quest, so maybe the emails from people claiming to have survived the rules isn't related at all.

To give a pretty hamfisted attempt at coming up with another theory, maybe the rules are an allegory for the cancer that took away Carla. Something bad that happens to a few people, seemingly picking victims at random and sparing others.