I'm trying to remember the name of a novel I read as a pre-teen. If I recall correctly, it would have come out in the 90s (though it may have been a reprint).
The novel is set in modern times. I recalled this because I've been reading a lot of urban fantasy lately, and this may be the first 'urban fantasy' book I read - our world, but with a touch of magic.
There's a somewhat clumsy, out of shape teenaged (or almost teenaged) boy who doesn't have a father. His father had been a stage magician who was constantly frustrated by his son's complete inability to preform stage magic, despite a prediction saying he would have a great or powerful magic. His father had died before the book started, I don't recall how long before.
The boy does something nice for an old woman (I don't recall what) and she gifts him a magical potion. A drop of it, rubbed on his shoulder blades, will make him grow wings.
The book follows him for the summer, as he spends his evenings flying. He makes himself a toga-like shirt out of some old curtains and spends time flying around at night.
I recall one scene where he learns about two children out in the middle of the bay (perhaps the book is set in San Fran? I don't recall) in a rowboat with one oar. He saves them (by pulling them back to shore using a rope they had with them) and gets mentioned in the paper as an angel.
At the end of the book, the potion has run out. There's only a tiny little bit left, and when he rubs it in, it creates only a single feather. Around that point, the boy realizes that he's no longer getting winded going up several flights of stairs and he's gained a lot of self-confidence.
From what I remember of the copy I read, the cover featured a shirtless boy wearing jeans standing in front of a mirror, looking at the large white feathered wings that had grown from his back.