There are 4 books in the The River of Time series. The newest book is Deluge which came out in 2014.

Gabriella has never spent a summer in Italy like this one.

Remaining means giving up all she's known and loved . . . and leaving means forfeiting what she's come to know--and love itself.
Most American teenagers want a vacation in Italy, but the Bentarrini sisters have spent every summer…
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2
Cascade

Book cover of Cascade
Mom touched my underdress--a gown made six hundred years before--and her eyes widened as she rubbed the raw silk between thumb and forefinger. She turned and touched Lia's gown. "Where did you get these clothes?"

In Cascade, the second book in the River of Time Series, Gabi knows she's left…

3
Torrent

Book cover of Torrent
When Gabi and Lia finally learn to surf the river of time, they realize they must make hard choices about life and love in Torrent, the third and final book in the River of Time series.

Gabi and Lia Betarrini have learned to control their time travel, and they return…

4
Deluge

Book cover of Deluge
It’s one thing to battle for life… WHAT DO YOU DO WHEN DEATH ITSELF COMES HUNTING? The Bettarinis and Forellis have found rich fulfillment, together in medieval Italia. But after fighting off countless enemies, they now must face the worst foe of all. As the Black Death closes in upon…