Storywalker

Part-time librarian Molly Matthews lives with the painful and exhausting chronic illness ME. The condition has broken every part of her life, except that she has plenty of time to escape into books. Especially her favourite fantasy series, Tamass the Fearless. And when Molly escapes, she really escapes, because she possesses a rare talent that allows her to enter a book and meet its characters and share their adventures.
Molly is a Storywalker. Novelist Paul Best doesn't walk in stories, but he's always been good at making them up. At least, that's what he believes, until he learns that he's spent years unconsciously channelling the life of his unknown twin brother Tamass as the hero of his successful series. When Tamass turns up at Paul's door one dark and stormy night with murderous demons on his tail, it's the start of a frantic multiverse-hopping adventure.
And then there's Molly, one of Paul's loyal readers, a woman who is so quietly ill in one world that she's nearly invisible, but who in other worlds is seen riding a warrior dragon.