This is a fantasy book in the sense that a modern day girl/teenage girl is a witch – and that's basically all I remember of the plot.
The cover was simple – a teal green background with a ghostly circle in the center of it, all wispy and stuff. I remember it being something like Ghostball, but I've searched that over and under, so that's wrong.
The series is meant for young adults and it MIGHT part of a series.
While it is a long shot, the series Spooksville may be what you are looking for, especially if you don't really remember many of the details.
Many of the covers match (Most are wispy, quite a few are green. A few have actual circles, but most are "circled" in the center of the books.)
Sample covers shown below:
There is a witch as a recurring character in the town, but one of the main characters is named "Watch".
Could it be the Doomspell trilogy by Cliff McNish?
In a blaze of light, rush of wind and scrabble of claws, Rachel and Eric are ripped through the wall and hurtled on to another world. Like thousands of other children before them, they have been snatched away by the Witch.
But this time the Witch has met her match. Rachel discovers that she has extraordinary gifts: she can transform herself into a feather, or fly on an owl’s back, just as the Witch can. The Witch is excited she has found someone to use for her own evil purposes. But for the Witch’s victims, Rachel is their only hope.
