New York is a magic city where almost anything can happen -- especially if you live on the 13th floor of an apartment house on Central Park West. Now of course, since most people are superstitious there is no real 13th floor in hotels or apartment houses, and the one where Ronnie and Gillian live, although it is right above the 12th, is called the 14th. It is not until the day they find the black cat Merlin that they discover the magical 13th floor where the hall paper is a pattern of bats, owls and broomsticks, with borders made of old charms and incantations. In the various apartments on this floor live a remarkable group of characters that the children meet and then see more of in the adventures that follow on the 13th of every month. Like all New York children, Ronnie and Gillian play in Central Park, ride on the ferry, and visit the fascinating shops near Broadway. But not all children are lucky enough to have a little box of daylight saving time to open in fog, and not all New York children can whistle up a snowstorm that falls only on Central Park while the rest of the city is bathed in dazzling sunlight, or ride with the Comet cleaners through the sky. Their mother could never understand why such extraordinary things happened only to the Saunders children, and not in other families. "Perhaps they do, my dear," their father told her. "Perhaps they do and the other people just aren't telling!"
Some of you may enjoy it via your public or university libraries. For example, Chicago and Evanston have it. It's not at any of my local libraries, alas. You can check for it here: http://www.worldcat.org/
It also has a sequel which I never got to read, entitled The Summer is Magic.