I found book two of the Up-and-Under to be a delightful continuation of the first.
These books are (as I understand) referenced in McGuire’s Middlegame, in much the same way as Fillory in Lev Grossman’s Magician’s. I have not read Middlegame, nor anything of McGuire’s aside from short stories in a few anthologies. I didn’t even know A. Deborah Baker was a pen name when I read the first book. So I can’t really speak to how this reads for fans of Middlegame.
This book provides more of what I loved about Over the Woodward Wall: a delightful middle-grade story of two children who found their way to Faerie by climbing over a wall that wasn’t supposed to exist (instead of going out the back of a wardrobe, through a looking glass, or down a rabbit hole.
It’s book 2 of the series, with book 3 coming out in October according to Goodreads (take that for what it’s worth). I don’t know if book 3 is the end or it’s going to be longer than that. If you liked Over the Woodward Wall I expect you’ll like this one. If you didn’t like OtWW (and opinions seem to be divided) you’re not going to find anything here to change your opinion as far as I can tell.
But for me, the series is a delightful return to the feelings of wonder I remember feeling curling up with Narnia as a kid. They’re novella-length, so they’re not big time commitments, and I would say that picking up book 1 is a very good investment for the time.