Okay so I apologise for thread-jacking but the story of how 50 Shades got published is WILD and an object lesson in evil genius marketing.
As you may or may not know, 50 Shades of Grey was originally written as a Twilight fanfic and posted online under the title Master of the Universe. This was at a time when fandom culture was at its height. EL James was what is known in fandom circles as a "BNF", or Big Name Fan, a fan who is extremely active and popular/famous within that fandom.
EL James spent several years assiduously building up her status as BNF, and her reputation as a writer of decentish (by fanfic standards) and explicitly sexual Bella/Edward fanfiction. Master of the Universe is what's called an "AU" or "Alternate Universe", a fanfic that takes pre-existing characters but puts them in a new realm. So in her AU, Edward was a billionaire businessman into BDSM.
Then she announced that due to the response to her little fanfics, she was going to convert her most popular fanfic Master of the Universe into a proper book. She changed the names from Bella and Edward to Anastacia and Christian, changed the title, changed a few tiny other details and self-published it on Amazon.
Now it's extremely easy to make the Amazon best seller list, if you're savvy about how Amazon's algorithms work, and have a decent amount of friends willing to shell out 99p for your ebook. Many members of the Twiight fandom bought her ebook even though presumably they'd all already read it for free when it was a fanfic, in order to support one of their own.
EL James had substantial media connections and media savvy due to her father working in TV, her own lengthy career working behind in the media, and her husband's career as a screenwriter. She launched a huge PR blitz trying to attract the attention of any publication she could find with her story of "the little self-published ebook that topped the Amazon best seller chart." This pretty shameless self publicity generated a fair bit of press which resulted in her book being picked up by a trad publisher, and the whole thing snowballed and snowballed.
Some time later (before she'd become huge) EL James' former assistant/helper leaked her emails online, and the emails showed that the entire Twilight fandom thing had been one big PR stunt from the start, a highly calculated attempt to infiltrate and monetise fandom. In the emails James talks dismissively of Twilight fans and openly admits she's only spending time writing fanfic and becoming a Twilight BNF because it's an easy way to latch onto a built-in audience base. One of the things new writers struggle most with is getting their names out there and building up an audience. By linking her work so explicitly with Twilight, pretending to be a Twilight superfan, and engineering conditions so that a good percentage of Twilight online fans felt honour bound buy her book, she leapfrogged that entire arduous process. She didn't need to build up her audience, she just helped herself to Stephanie Meyer's audience instead.
Obviously something about her book struck a chord with people. But to invent such a calculating plot was pretty devious, and she certainly has a genuine gift for PR if not for writing.