It's a very quick read (less than an hour) and I've spent more time sitting here trying to write a competent review than it actually took to read the book.
So, first: read this book. Read it especially if you didn't grow up around anyone who is non-binary. Read it especially if you're interested in reading a vulnerable, well written coming-of-age story that is probably very different than your own. Read it if you want to get a deeply human understanding of someone trying to find their place in the world in a way that goes far beyond what the clinical phrase "gender dysphoria" can convey.
One of the things I detest about the ongoing culture wars around gender is that the "culture war" too often ends up substituting for knowing real live, complex, human beings. I highly recommend meeting Maia Kobabe, who loves Bowie, and Gaiman, and Pratchett, and Wilde 🙂
There is nothing in this book that is going to shock the sensibilities of any contemporary high-school student, and is unlikely to shock the sensibilities of any bright 8th grader. If you're going to hand it anyone younger than 8th grade, give it a read yourself first, so that you (for instance) have a ready explanation when asked what those vibrating egg-shaped things do 🙂
I've seen a lot of reviews encouraging people to give this book to any kid they suspect might be non-binary, or trans, or otherwise "queer" in some way. That may well be good advice, but my personal recommendation is to hand this to any kid in which you hope to instil a sense of understanding and empathy for people whose experiences are very different from their own.
One item of note: the author of this book, Maia Kobabe, prefers pronouns that aren't common English words. I know me... I am unlikely to memorize non-English pronouns, but I think I can meet the author at they/them. (If you're considering lecturing me on that, don't bother) The author is very uncomfortable with being referred to by feminine pronouns, so I'd appreciate everyone keeping that basic respect in mind.
One last thought: if you have children older than 12 (or so) and are thinking to yourself "I don't want my child to learn about trans or non-binary people!"... it's too late. They're going to... on the internet, from their friends, from media, they're going to. Your only choice now is whether they learn to view trans and non-binary folks as worthy of fear and contempt, or worthy of understanding and human decency.
I hope you'll choose the latter.