A girl (Girl A) lives with her parents in a brownstone(?) or apartment building across the street or adjacent from Central Park. She has a good life but would like a friend of similar age. Very early on in the book, a girl about her age moves into the building! Girl A makes friends with this girl, has many high spirited adventures in and around Central Park & NYC.
This book is titled something like “The World of Xanadu” or “Xenon” or “Xenia”, but “Xanadu” sounds most right to me. the X-word is the title of the imaginary world that the girls have made up. Early on, Girl A daydreams that she will find a friend her age and when she does, she says “We will make a fantasy realm called ‘Xanadu'”. This aspect feels like it has a “Bridge To Terabithia” element to it.
The setting is very close to “The One Hundredth Thing About Caroline” by Lois Lowry, but it isn’t that, because that book has more of a mystery/suspense nature to it.
Any help would be wonderful! Thank you!