Two YAs in a row for the group, but I don't think that's way only two of us liked it. The book grabs parts of lots of other books and TV, even old parts, and while some parts are cute, most of it just reads like a collage.
Two girls in London are pulled through an odd place into UnLondon (Un Lun Dun). It turns out that there's lots of cities underneath, like Parisn't, but in unLondon, the Smog and an unbrella (umbrellas are in London) guy are trying to take over the city. They're being helped by the UK Minister of Environment -- she's sending the smog down to unLondon so the Smog can get bigger and she can be Prime Minister. She's not smart enough to think that the Smog will come up and take over London when it's done with unLondon.
It turns out that one of the girls is the Chosen and has to get seven articles in a quest to save the abcity. She goes home and doesn't remember anything. The other girl, her friend, is still worried about the abcity and goes back. She takes over the quest but decides it will take too long so she only gets the first item and then just finds what the quest is supposed to give her. She uses it to try to get her small group where they can stop the Smog and the unbrella man, and eventually, they win. She goes back home, but plans to go back to unLondon soon.