I just looked through all the posts here with the tag "ep:the monster at the end of this book" and didn't find this, so hopefully someone out there will recognize it. I had started reading it but my Kindle crashed and I couldn't find the link again...
I can't recall if it was Sam/Dean or J2 but after the episode, they started reading fanfic. At first it was the really out-there stuff - mpreg, bizarre AUs - for the lulz, but one of them started finding good relationship porn which made him start to see the other differently. And that was where it dropped me. I checked all the stories linked in previous posts and it's not any of them...does this ring any bells?
ETA: Found it!
Let's Get Wrecked on Poptarts and Sex by
queenklu - it was read aloud as part of the Help Haiti auction, and was posted yesterday, and the description rang MY BELLS!!!! Just in case anyone else is interested, it's a hoot and it is J2, Jared's POV as he finds he really does want from Jensen what he sees in the fanfic.