This is all her website says:
London, Manchester UK
I assume people have to keep an eye open and an ear to the ground to find out exactly where, but I have some info on a London signing from Becky of The Bookette:
Golden Treasury Shop, Wandsworth
4.15 - 5.30
ETA: Becky has some more info about the London signing:
I may or may not be going to this signing, I haven't decided yet. But it's awesome news, isn't it?!
Maggie Stiefvater will be signing books on October 15th at The Golden Treasury Bookstore in Wandsworth. It starts at 4.15 and Maggie will give us a talk about her books before she signs. Anna, Assistant Manager at Golden Treasury thinks there will be a chance to ask Maggie questions after the talk and before the signing.
I can also tell you that if you can't make the London signing, Maggie is also going to be at the Cheltenham Literature Festival on Saturday 17th October when she will be in discussion with Vampirates author Justin Somper. This event starts at 5.15 pm and lasts an hour. Click here to find out more.
If anyone knows of any other dates, times and places, please let me know so I can update this post. It would be good if we all knew.