(Covers lead to reviews on my journal.)
From the back cover of Blue Magic:
This powerful sequel to the A.M. Dellamonica's Sunburst Award–winning contemporary fantasy Indigo Springs starts in the small town in Oregon where Astrid Lethewood discovered an underground river of blue liquid--Vitagua--that is pure magic. Everything it touches is changed. The secret is out—and the world will never be the same. Astrid’s best friend, Sahara, has been corrupted by the blue magic, and now leads a cult that seeks to rule the world. Astrid, on the other hand, tries to heal the world.
Conflicting ambitions, star-crossed lovers, and those who fear and hate magic combine in a terrible conflagration, pitting friend against friend, magic against magic, and the power of nations against a small band of zealots, with the fate of the world at stake.
Blue Magic is a powerful story of private lives changed by earthshaking events that will ensnare readers in its poignant tale of a world touched by magic and plagued by its consequences.
The Door into Summer is one of Heinlein's earlier works, whose protagonist Dan, shuttling between time, is looking for his 'door into summer' alongside his cat Pete. An engineer working with household robots, he's on his way to a successful life inventing new things when he's betrayed by his partners.