The Secret Commonwealth is the second part of The Book of Dust. The central character, Lyra Silvertongue, once known as Lyra Belacqua, was also the protagonist of a previous trilogy, His Dark Materials: in fact, her name was both the first word and the last in that work. In that story, she was about eleven or twelve years old.
In the first part of The Book of Dust, La Belle Sauvage, Lyra was a baby. Although she was central to the story, most of the action of that book concerned a boy called Malcolm Polstead, himself about eleven years old.
In this book, we skip ahead twenty years or so. The events of His Dark Materials are ten years in the past; both Malcolm and Lyra are adults. The events of La Belle Sauvage are even further back in time.
But events have consequences, and sometimes the effects of what we once did take a long time to become fully apparent. At the same time, the world moves on; power and influence shift, or increase, or diminish. And the problems and concerns of adult people are not necessarily the same as the ones they had when young. Lyra and Malcolm, as I say, are not children anymore.
Philip Pullman