His Dark Materials 3: The Amber Spyglass by Philip Pullman.
Along with the return of Lyra and other familiar characters from the first two books come a host of new characters: the Mulefa, mysterious wheeled creatures with the power to see Dust; Gallivespian Lord Roke, a hand-high spymaster to Lord Asriel; and Metatron, a fierce and mighty angel. So too come startling revelations: the painful price Lyra must pay to walk through the land of the dead, the haunting power of Dr. Malone’s amber spyglass, and the names of who will live – and who will die – for love. And all the while, war rages with the Kingdom of Heaven, a brutal battle that – in its shocking outcome – will uncover the secret of Dust.