1.) One word: Centaurs. When Regan, a horse loving young girl, is transported to a world of centaurs, she's finally found a place she belongs. I've read plenty of fantasy books where centaurs are mentioned and maybe one or two where a minor character is a centaur. In ACROSS GREEN GRASS FIELDS, we get to see a whole world of centaurs and what their lives are like in the Hooflands. It's creative and mesmerizing.
2.) This is a perfect stepping point into the series. The Wayward Children series follows a pattern of past and present in the book releases. The first book takes place in the present, focusing on a school for wayward children who have returned from portal worlds. The second book provides the background of a character, telling the story of their own journey into their portal world, and the series continues alternating. In the first 5 books, you meet new characters, but the central group is largely the same. In ACROSS GREEN GRASS FIELDS, you get the background of a new-to-the-series character, Regan. It is likely she will meet the central group in book 7, but for book 6, you don't have to have any prior knowledge of the series going in. It's a great introduction if you want to see if you would like the series and want to commit to reading the prior books (it should be noted that each book is a novella, so the books aren't terribly long).
3.) The themes of found family and belonging- While growing up, Regan is desperate to fit in. She sees what it's like when you don't fit a very specific mold of what a 'girl' should be, and it scares her. The only time she feels truly comfortable and herself is when she is with horses. There is so much beauty when she crosses world and finds a place to belong. Ironically, in the Hooflands, humans are rare and singled out, yet it is in that space where she finds a family who treats her with respect and love, as one of their own. I loved her adventures and the way she grew into herself in her new home.