In the Garden of Iden (The Company, 1)

This is the first novel in what has become one of the most popular series in contemporary SF, now back in print from Tor. In the 24th century, the Company preserves works of art and extinct forms of life (for profit of course). It recruits orphans from the past, renders them all but immortal, and trains them to serve the Company, Dr. Zeus. One of these is Mendoza the botanist. She is sent to Elizabethan England to collect samples from the garden of Sir Walter Iden.
But while there, she meets Nicholas Harpole, with whom she falls in love. And that love sounds great bells of change that will echo down the centuries, and through the succeeding novels of The Company.