I saw a similar link t'other day (I'll see if I can find it) that was the same story, except different. Rather than a reference book, which I do believe she's already writing, she said she'd love to write an eighth book in which Harry is
not the main character.
Which I've just realised is what Jaire said in fewer words in his first post.
With those details, I see it less as a spin-off, and more as a novel set in the same Universe.
EDIT: Here's the link I found.
JK Rowling drops hints of possible eighth Harry Potter book | the Daily Mail
A big IF, and one that might come to fruition 10 years down the line.