Deeper, Darker, Older by P. J. Richards

Ancient yew trees - living links between the past and present - embody a magical discipline that offers a way to reach beyond the mundane world. An occult group has rediscovered this knowledge, and uses it to experience a reality where legends live, magic is currency and willpower is a weapon. They find themselves drawn into a hunt for the last magical artifacts left in Britain - but the land is an entity with its own deadly agenda, and is harnessing these individuals for its own ends.