Empire Games by Charles Stross
Empire Games is the first book in the exciting series set in the same world as Charles Stross' The Merchant Princes series.
It's a time of ambition, treachery and dangerous secrets . . .
Rita Douglas is plucked from her dead-end job and trained as a reluctant US spy. All because she has the latent genetic talent to hop between alternate timelines - and infiltrate them. Her United States is waging a high-tech war, targeting assassins who can move between worlds to deliver death on a mass scale, and Rita will be their secret weapon.
Miriam Beckstein has her own mission, as a politician in an industrial revolution US. She must accelerate her world's technology before their paranoid American twin finds them. It would blow them to hell. After all, they've done it before. Each timeline also battles internal conspiracies, as a cold war threatens to turn white hot. But which world is the aggressor - and will Rita have to choose a side?