Mary must discover what is wrong with the trees, and to do so, she must create the Amber Spyglass of the title, a tube with lenses created from a local resin that allows one to directly view 'dust'.
With the Amber Spyglass, Mary is finally allowed to see that the flow of 'dust' in the world she is visiting has been effected by the holes cut in the fabric of the Multiverse by the Subtle Knife.
Pullman shows his extraordinary imagination in The Amber Spyglass, with physicist Mary Malone's sojourn among the Mulefa.
The Amber Spyglass is the third and final installment of Philip Pullman's His Dark Materials trilogy.
Amber Spyglass' ending is quite one of the most poignant of any I have ever read.
Now, in The Amber Spyglass we discover the true nature of angels.
Early in the morning of the twelfth of June he came out of his tent, which was pitched that day on the steep left bank of the Niemen, and looked through a spyglass at the streams of his troops pouring out of the Vilkavisski forest and flowing over the three bridges thrown across the river.