While the title of this book appears to make this a sequel to Twenty Wishes, therefore putting it between that book and Summer on Blossom Street, be warned , this book should follow Summer making it #6.5 not 5.5. I really don't think that it tells a complete story but instead follows threads begun in Twenty Wishes and Summer on Blossom Street. In some ways this is more of a completion of those stories than a story in it's own right. I recommend reading this after both Twenty Wishes and Summer on Blossom Street. Also there are characters whose backstories are in earlier books, especially Lydia, but those backstories aren't really significant in this book, except for Anne Marie and Ellen.
This is a little more than a short story and maybe enough for a novella. But there was way too much info dump which are needed to fill in an obvious gap between Twenty Wishes and this book. But much of that is covered in Summer. (I followed the numbering sequence for the books listed in the title section on Goodreads and therefore read this before Summer.) The whole things seemed a little rushed since I read it without Summer. For one thing, too much of Tim and Anne Marie's relationship was skimmed over, especially the earlier part. All of the part about how Tim comes into their lives is backstory which is covered in Summer.
A common problem for me with romance novels is the idea contained in so many that "love" is all about warm and fuzzy feelings. Being "in love" seems to be so important. I believe that love is an action verb. It's a choice. This story is interesting in that Anne Marie seems motivated by the warm and fuzzies, but she admits what Lydia tells her that love is a decision.
Mature themes: nope - none, unless you count some heavy kissing, which I don't.