His Dark Materials trilogy by Philip Pullman. Fun to read, but a little weak. The knife and the bear are cool, but on the other hand some parts of books 2 & 3 felt too sappy. The mixing of the worlds, and reasoning behind the whole business, are what felt weak to me. But then again I saw this on my daughter’s 4th-grade recommended-reading list, so “a little weak” is totally appropriate strength depending on the audience!