Teenage girl accidentally turns herself invisible with a magic device.

Longer: This was a children’s/YA book I read in the early ’90s at the absolute latest. I’m pretty sure it was a hardcover, and the title was some common phrase like “Vanishing Act” which also referred to magic or invisibility, and the letters on the cover were gradually vanishing, from white into the black of the cover.

The main character was a girl, at least young teenage, who was a stage magician. She was described as having large, quick hands, which made it easier for her to do card tricks. She frequented a magic shop in her town and one day found a boxed magic trick which the owner didn’t want her to buy; she might have either stolen it or left the money and slipped out when the owner wasn’t looking.

The trick was a box with a row of buttons down the exact middle of it, six or seven in rainbow order, with the last one being either white or black. There was a sheet of directions and a piece of cloth included in the box, and when she put the cloth on something and pressed the buttons in the order specified in the directions, the something turned invisible (really, truly invisible; not a trick). She then followed the directions and made the thing visible again.

She used the box as her grand finale in a magic show she did, and the store owner was really angry at her, partly because it was cheating and partly because maybe there was something wrong with the magic box itself.

The accident happened when she tried it again, and it turned out her knee was touching the corner of the fabric, and the directions were folded up in her pocket and were also turned invisible, and so was the magic box itself.

She finally figured out/half remembered the right order to press the buttons in, and then it didn’t work, but she turned the box around (since it was perfectly symmetrical) and it worked.

There was a bit of scariness when the magic store owner (cranky old guy?) was trying to get the box away from her–maybe he fell in the water? Something about a warehouse? This bit is all very fuzzy.