I read Frances Hardinge's Gullstruck Island (retitled The Lost Conspiracy in the US) and liked it a lot. I want to read her other novels too.
I've just finished Jenny Valentine's fourth novel The Double Life of Cassiel Roadnight. For all the humour - in the narrator's voice - in her first novel Finding Violet Park (which won the Guardian Fiction Prize) there was darkness there, and it's also there in her subsequent novels Broken Soup and The Ant Colony. It's even more so in this one, which becomes a fully-fledged crime novel, influenced by Josephine Tey's 1949 novel Brat Farrar - as Valentine acknowledges in her author note. Each one of her novels is structured around a mystery that needs to be solved.
A boy in care is identified as the missing teenager Cassiel Roadnight (he looks just like him) and, out of a sense of wanting to belong, goes along with this even though he knows he isn't him. Cassiel's family accept him, although they're clearly hurt by his disappearance for two years without explanation. But "Cassiel" is afraid of what would happen if they find out, or if the real Cassiel turns up? As "Cassiel" finds out about his past, he realises he is in danger.
Needless to say, this is less light-hearted than Valentine's earlier novels, but her strengths are evident too. She's very good at family/sibling relationships, in this case particularly dysfunctional ones, and her dialogue always rings true.
(I haven't read Valentine's books for younger readers, Iggy and Me and its sequels. I have read Ten Stations, which was a short story for World Book Day, featuring some of the characters from Finding Violet Park - it's pleasant but nothing special.)