A note for posterity : this review was written roughly a week after publication, April 1, 2020. The year that March came in like a Lamb and went out like a Lion. New York City is currently the epicenter of the Covid-19 virus in the United States.
And, I say that because the primary storyline of "The City We Became" involves a multi-tentacled foreign invader attaching to various surfaces, popping out of people's skin, and disrupting traffic and general way-of-life. It is variously likened to an afro, or a porcupine, and it is going to require New Yorkers coming together to stop it.
And, I know that the central metaphor is for something completely different -- when we meet the avatar of the invading whiteness she is calling the police on two non-white people in the park; one technique for keeping the white at bay is literally throwing literal money at it. Hey, as NK Jemisin said in her 2018 Hugo award acceptance speech, "[I've been] advised to tone down my allegories. . .I didn't." -- but I'm reading a book here while I'm essentially locked into my house in my city (Baltimore) and the similarity can't be ignored.
That's one thing good books and good stories do : they make the specific universal. And so this book lets me consider the coronavirus, and stitch New York onto Baltimore and consider where we are living and dying and being invaded. . .and I can see the battles between the "real" Baltimore and the white tendrils that I can view from my front porch. And I know "my" Brooklyns and Bronxes and Staten Islands.
So the book itself is an urban Lovecraftian horror(?) story. Also reminiscent of Jeff Vandemeer (more specifically the Ambergris stories), but Lovecraft plays a large role here. . .references to the non-euclidean geometries, Lovecraft's places, and HP himself abound. This book is in a dialogue with his writings, not overtly, and adds just another spice to the stew.
At the top level, this book is a romp with a "we're getting the band back together" vibe. It is filled with various side-quests, while always moving inexorably towards the final Boss fight. I must say that I was hoping for some kind of awesome robot "Form Voltron" moment, but what we get is more true and sensible.