No More Magic by Avi, 1975.

Chris has a fun time on Halloween with his best friend, Eddie.  The two of them are dressed as superheroes.  Eddie is Batman, and Chris is the Green Lantern.  However, the next morning he discovers that his bike is missing.

Chris loves his bike because of its wonderful green color.  He thinks of green as a magical color, and the bike has a great shimmery kind of green paint that reminds him of the Green Lantern.  Although his green bike was a little old and his mother gets a good deal on a newer yellow bike, Chris misses his old one and begins investigating, trying to figure out who could have taken it.

At first, the challenge seems difficult.  On Halloween night, lots of people were running around in costume.  However, Chris’s community isn’t too big, and he is able to identify many of the people who came to the house around the time that the bike disappeared.  One kid that he doesn’t know well is Muffin, a new girl at school, who came to the house in a nurse costume.  There was also a mysterious kid who came in a warlock outfit, with a cape and a tri-cornered hat.

When Chris interviews Muffin to see if she knows anything about his missing bike, he learns that the warlock costume originally belonged to her.  She had made it herself and was proud of it, but someone stole it while it was hanging up to dry.  The nurse costume was a last-minute substitute.  Chris thinks that whoever was wearing the warlock outfit is the most likely suspect for stealing his bike because the local police chief told him that a man had complained that someone riding a bike like his and wearing a costume like the warlock costume ran into him.  He suggests that the two of them team up to find the mysterious warlock/thief and get their stuff back.

Muffin is kind of a strange girl and is oddly evasive about her life and family.  At first, all she will tell Chris is that her real name is Maureen (but she doesn’t like that name) and that she has come to town to live with her aunt because her parents “aren’t around.”  Her aunt seems somewhat strict, and Chris donates some of his own money to Muffin to help her buy a used bike so the two of them can ride around together, looking for clues.

One of the first things they learn is that the person wearing the warlock costume won the prize for the best costume at the Halloween parade downtown.  Chris didn’t attend the Halloween parade, but his friend, Eddie, says that he went.  However, Muffin and Chris notice that Eddie’s name wasn’t on the list of attendees.  When they try to ask Eddie about it, he becomes strangely angry.  Chris doesn’t want to think that his best friend could have stolen his bike or the warlock costume or both, but why would he try so hard to avoid answering their questions if he didn’t have something to hide?

Then, Chris learns why Muffin is so secretive about her life.  The school counselor has noticed that Chris has made friends with Muffin, which she thinks is good, but she tells Chris’s mother about Muffin’s circumstances because she thinks that they should know.  Muffin’s parents recently split up.  For unknown reasons, her mother walked out on her father.  Muffin’s father was distraught when his wife left, and he decided that he should go after her and try to straighten things out with her.  He left Muffin with her aunt and went to try to find his wife, although no one really knows exactly where they went or when Muffin’s father might be back to get her.  The part that worries the school counselor is that Muffin seems to be blaming her parents’ problems and their mysterious absence on magic.  She thinks that magic made her mother go away and that magic is what’s keeping her father away.  The school counselor thinks that being friends with Chris, who is a pretty practical kid (even though he also believes in magic a little), will help bring Muffin down to Earth, and Chris’s mother encourages him to be nice to Muffin and bring her over for dinner.

Still, Chris can’t shake the thought of magic from his head.  He has the strange feeling that, somehow, his magic bike and the magic that Muffin is looking for to get her parents back are connected.  One night, he even thinks that he sees the warlock flying across the sky with his bike, although his father thinks that he dreamed the whole thing.  Neither of his parents believes in magic, and Chris even comes to question whether Muffin is to be trusted.  However, as Chris’s father says, it’s often a matter of asking the right questions before ending up with the wrong answers.

Muffin’s belief in magic comes from what her father said after her mother left.  He said that “the magic had gone,” and he blamed himself for being too self-centered to realize it.  Adults would realize that “the magic” the father was talking about is the romantic feelings that come with love, but Muffin had interpreted the phrase literally.  When her father decided to go after her mother and try to work things out, he said that he was going to “go and find the magic and make it work again.”  Again, Muffin believed that was literally what he was going to do.

It turns out that there are perfectly logical explanations for what happened to both Chris’s bike and the warlock costume, and the two missing objects are not necessarily connected.  Much of what happened on Halloween night involved a series of coincidences, deceptions, and misunderstandings, but even when the full truth becomes known, Chris and Muffin can’t shake the thought that there was a kind of magic behind it all.  Plus, as the kids had guessed, Muffins parents return when they find Chris’s magic bike.

The book is available to borrow online for free through Internet Archive.