'Dreamstone Moon', written by Paul Leonard, features the Eighth Doctor and Sam.
Back Cover Blurb
Sam is on her own, but her distance from the Doctor doesn't make for a trouble-free life. Rescued from an out-of-control spaceship, she finds herself on a tiny moon which is the only known source of dreamstone, a mysterius crystalline substance that can preserve your dreams - or give you mightmares.
Pitched into the middle of a conflict between the mining company extracting dreastone and exological protesters, Sam thinks it's easy to decide who the good guys are - until people start dying, and the killers seem to be the same species as some of her new friends.
Meanwhile, the Doctor has tracked Sam down, but before he can reach her he's co-opted by the Dreamstone Mining Company and their sinister military advisers. Suddenly, it's war - and the Doctor is forced to fight against what he believes in. He alone suspects that dreamstone isn't what it appears to be. But nobody's listening - and nobody could dream who the real enemy is...