What happened after the end of The Amber Spyglass? Sadly, our beloved Philip Pullman has said there will be no direct sequels now that the trilogy is over: Lyra and Will's story is over. However, hope remains:
Philip Pullman is working on a sort of companion novel to His Dark Materials, entitled "The Book of Dust."
This will contain various short stories about many of the secondary characters in the series, such as Lee Scoresby and Serafina Pekkala. There may also be information about Dust, the Alethiometer, the Subtle Knife, and dæmons - all the things we wondered about, but weren't important enough to put into the books themselves.
Despite his planned intentions, Pullman did indeed find himself writing about Lyra again. This short story, Lyra and the Birds, developed into the publication of Lyra's Oxford, a unique book containing the short story itself and several 'materials' from Lyra's world. Click the previous link to read more about it. Pullman has stated that he took this story out to prevent it overwhelming the other short stories in The Book of Dust. However, it appears from quotes and interviews that Pullman is merely using Lyra's Oxford as a bridge to a larger story with her in the Book of Dust. The details of that story have not been mentioned, but the events of Lyra's Oxford supposedly set the scene, and introduce us to necessary characters in the new story. It seems Pullman cannot let Lyra go just yet.
Unfortunately, since the publication of Lyra's Oxford, there has not been much talk of the Book of Dust from Pullman. He has refered several times now to be "just starting" the book, each repetition of this several months down the line does not bode well for an early release.