In The Golden Compass, book 1 of this series, we met Lyra, the half-wild charge of a scholar of an Oxford University in a parallel universe, and follow her adventures with warrior polar bears to the gates of a new world. This is book 2 of the series, in which Lyra meets Will, who had fled 'our' world into a parallel world to escape shadowy men who are pursuing him to obtain the papers of Will's long-lost explorer father. As Lyra and Will explore the third parallel universe, we learn of the titular knife, which can cut any substance, including the membranes that separate parallel universes. We also learn of the epic battle playing out across the universes, which involves god and angels, and both of their fathers, and of which they're unwittingly key players. This is a fantastical, imaginative adventure spanning many climes and crossing the boundaries of the universes.

I was pre-reading for a pre-teen, with an eagle eye out for too-adult themes. Although I didn't find any, there is a very intense last-stand of a beloved character, which might be too much for sensitive readers.