Backwards Day
One of the first books we found that talked about gender; I like this book. Set on the planet Tenalp where everything is backwards from the way it is on our planet, except for one day a year when everyone and everything is backwards. It is longer than it needs to be, but the story is cute and all the people are vibrant colors so it is not so very "white" like many of the books we have found. It is about a girl who longs to be a boy—and gets to!—so it is a rare book in this category, and it walks through the parents being confused and counseled by experts to accept their kid.
From the Publisher:
BACKWARDS DAY, set on the planet Tenalp, introduces us to a world where there are seventeen seasons, including one where bubblegum falls from the sky for three days and a single day when everything - everything everywhere - is backwards. Andrea looks eagerly forward to Backwards Day every year, so she can turn into a boy for the day. But one year she doesn't turn along with everyone else. She's miserable. The very next day, however, she turns into a boy - and stays that way! He's delighted, but his parents are distressed, and take him to the big city to consult with Backwardsologists. When they finally figure out what's happened, the miracles of Backwards Day are fully revealed to the reader.
Written by S. Bear Bergman and illustrated by New York cartoonist KD Diamond.