I really liked this story.
It was wonderful to learn more about "Annie" and I really hope that her past with Elizabeth comes up with Toby at some point. I'd really like Toby and Co. to know that side of her.
I think it worked well as a short story but a part of me does wish that it would be expanded into a full novel. There's a short section on how alienated Elizabeth feels as she gets older and Annie stays the same age and I think to really feel her pain, sense that she doesn't belong in any community requires a longer piece. She sort of fits with the Selkies (which you don't see/read about) until as she gets older and she doesn't have a skin, she can probably pass for human and appear to fit in for a while while she and Annie are roughly the same age (though given the time period there would obviously be problems with that since she's part of a same-sex couple). Eventually she'd be ostracized by that community to as she appears to be increasingly older than her lover. And the fae community has never really accepted her because she's a Selkie without a skin. So by the time she's offered a skin it's not just a skin but a chance to belong somewhere again. Belong to a people/group rather than feeling like she only belongs with one person but is alienated from everyone else around her. I would have liked to see more of that, and maybe more recognition from Annie that it was a problem in the first place and the role it might have played in her decision making. Not that it ultimately would have made a difference, understandably.
I think at one point I'd known the origin of the Selkies in McGuire's world but I'd forgotten it until reading this short story.
McGuire already has a large number of irons in the fire or I'd think more about wondering if the idea of a novel version appeals to her.
I like Elizabeth a lot and I liked seeing Annie through someone else's eyes. Someone who loves her and isn't afraid of her. Though I did wonder why Annie didn't bother to make herself look a older when they were out among the mundane population if it would help Elizabeth's feeling of alienation.
And I'd also have to give some thought to how well Elizabeth knows Annie when she doesn't know who she really is. And even if she loves Annie would she love the whole person if she were allowed to know the whole person rather than this fraction she's allowed to see/person Annie's constructed? There's a whole new level of complexity to explore.
I'd definitely be interested in reading more non-Toby stories about "Annie." (Not that I don't love the Toby stories).