The Shadows (Volume One of The Books of Elsewhere) by Jacqueline West stars Olive, 11, who moves into a strange old house with her very distracted parents – and this is one of those GU stories that takes place entirely within one house. Left mostly to her own devices, Olive soon discovers a magical pair of glasses that lets her enter the creepy paintings that hang in every room. A talking cat gives her a stern warning about her explorations, which of course she ignores. Then Olive meets Morton, a boy trapped in the paintings, terribly afraid of an unnamed man. Eventually Olive finds two more talking cats, all guardians of the house, and discovers the story of the original owner, who seems to be both the mysterious painter and the man Morton fears so much.
For awhile it seems like the cats may actually be against her, but instead it is another companion who betrays Olive, and almost destroys her while trying to resurrect the spirit of the old man. Ultimately, Olive is trapped alone in the attic, separated from her companions, and must face this adversary while he preys on all her fears. But fortunately she is clever enough to defeat him.
This is the beginning of a fairly long series, and it looks like the third book features a descent underground!