Magic in the Park, by Ruth Chew

Jennifer thinks she hates Brooklyn, until her mother suggests she go to Prospect Park. There she meets Mike, a neighborhood boy, and makes her first friend. She and Mike discover that Prospect Park is full of magic, from a mysterious man who likes to feed the birds to a disappearing tree.

I'd never read this book before, and it's one of Chew's best. She's wonderful at hitting on childhood "what if" fantasies, like "What if I could travel underground and see tree roots from underneath and pet moles in their burrows?" There's also a hilarious plotline in which Mike gets turned into a pigeon and he and Jennifer have a lot of awkward conversations with birds. The bird-feeding man's identity was unexpected and very numinous. And it all feels more real for being set in a city where you have to watch out for broken glass and bicycle thieves.