Background on what My Immortal is and why it's so popular - Vulture (from 2015)
Run-down of the evidence for authenticity - Daily Dot
Some details on the book - Buzzfeed
Amazon listing - coming 29 May 2018
My Immortal is a landmark in HP fandom and the broader fanfic community -- and the identity of its author is one of the longest running online mysteries, which has thwarted years of Internet detectives -- so this is fairly exciting news to someone like me who grew up with Internet culture.
In early August, "Tara Gilesbie" decided to come forward after a decade of radio silence since My Immortal was abandoned. She reopened an old FictionPress account and then posted a link to it on the My Immortal wiki under the name Fokenprepz. In late August, she posted another update to FictionPress refuting the theory that she was Lani Sanem, author of Handbook for Mortals, which was recently the subject of controversy regarding its inclusion in the NYT bestsellers list (previous GAF thread on the subject). The FictionPress was updated again specifically denying this and giving a few details about the author.
There have been numerous impostors claiming to be Tara over the years, so this was initially rejected as a hoax by many people, including the world's leading authority on My Immortal. However, he relatively quickly admitted that he had been mistaken and the new Tara was in fact genuine. Using information from the FictionPress account as well as a now-deleted tweet from an employee of St Martin's/Macmillan (the publishing house who will be putting out the memoir), tumblr sleuths identified YA author Rose Christo as the real Tara, which she has since acknowledged.
Ms Christo initially refused to give details but she has in the past couple days admitted that My Immortal was a trollfic which (a) settles one of the longest-running debates about the story and (b) makes it a legitimately brilliant work of parody. The memoir will cover a number of topics relating to Ms Christo's childhood aside from My Immortal which, from what she's mentioned so far, will probably be fairly heavy reading. But I'm thrilled to have seen an answer to a mystery that I never thought would be solved, and she seems like a pretty cool person, so I'm looking forward to reading the memoir when it's available.