I'm not sure I understand the hype of this book or how it went from fan-fiction to mainstream.
The book is essentially the most popular Twilight fanfiction that has ever been written... it is the most well-known Twilight story out there, written by essentially the most 'famous' author in the fandom. It apparently received over 15,000 reviews when it was posted online.
Unfortunately, the popularity of the story caused the author believe that it was good enough for mainstream publishing (despite the fact that publishing is generally considered a big no-no when it comes to fanfiction, due to the fact that it is a derivative work based on someone else's creation) and she decided to self-publish using an online vanity publishing house that specialises in taking Twilight fanfiction and publishing it as so-called 'original fiction' (which it isn't really, since it was written about already existing characters - just changing the names doesn't change the character traits).
I believe the mainstream hype is partly due to the author and her husband both working in the media business and knowing how to do good PR, and also partly due to the hundreds, maybe even thousands, of Twilight fans who have no doubt been spreading the word online.
Personally, I feel that the whole thing is morally wrong (and there might possibly be legal issues with it as well - that's a bit of a grey area though), as the whole point of fanfiction is that it is based on someone else's work and when you post it online, you include a disclaimer stating that you don't own the characters and that you have no intention of profiting from the work.