Diana Wynne Jones, The Merlin Conspiracy, 2003.
Jones was one of my favourite authors when I was younger, thanks to books like Charmed Life and The Lives of Christopher Chant. She's a brilliant, imaginative & exciting author--far worthier of being read than JKR, in my humble opinion, but oh well.
This latest novel twins the stories of Arianrhod & Nichothodes as they try to save the world of Blest, and generally make sense of their own lives. Like all of Diana Wynne Jones's books, the kids are far more willing to recognise the world for what it is while working for its betterment than the adults. Roddy & Nick have to make sense of their respective magics, and work together to stop the takeover by the evil person whose name I'm not going to tell you (so as to not ruin the surprise).
It's not her best book, but it's riveting & fun, & I like coming back to her writing.
Over a library in one of her stories, she put an inscription about the books inside that I think I'd like above my books one day: "Monuments more lasting than brass..."