Q:

A story about a man whose wife recently passed, living into a least plausible reality

I remember reading the entire story online. 
The man ends up at an old bookstore that sells pulp scifi that his wife liked, realizes some of the titles are fake. The owner gives him a cryptic book and disappears (dies?). The book is (potentially portrayed as pseudoscience) about how we don't die, simply continue jumping into the least plausible reality. 
He gets into an accident shortly after, and the story culminates decades (much longer?) later, with an alien species finding/using him as a repository of information about our species?

A:

"Divided by Infinity" by Robert Charles Wilson, full text at Tor.com.
This story was the subject of this old question and this one.
In fact there is a microgenre of quantum immortality fiction.

