Diana Gabaldon's newest in the Outlander series (Book 7) An Echo in the Bone is due to be published on September 22, 2009. If you like Historical Novels you will LOVE this series.
From her site:

The adventure began in 1991 with the classic Outlander ("historical fiction with a Moebius twist"), continued through five more New York Times-bestselling novels--Dragonfly in Amber, Voyager, Drums of Autumn, The Fiery Cross, and A Breath of Snow and Ashes--and a nonfiction (well, relatively) companion volume, The Outlandish Companion, which provides copious details on the settings, background, characters, research, and writing of the novels. Gabaldon (it's pronounced "GAH-bull-dohn"-rhymes with "stone") has also written two historical mysteries, Lord John and the Private Matter, and Lord John and the Brotherhood of the Blade, as well as several novellas featuring Lord John Grey (which will appear in volume form this November, as Lord John and the Hand of Devils). A Breath of Snow and Ashes, the most recent novel in the main Outlander series, opened simultaneously at #1 on the bestseller lists of four countries, and won both a Quill Award and the Corine interntational literary prize for fiction.

Previous books in the series are:
Outlander
Dragonfly in Amber
Voyager
Drums of Autumn
The Fiery Cross
Breath of Snow and Ashes

You will not want to put these down once you get started. I am going to start re reading them this summer since the last one was published in 2005. I am so excited!!

As a side note, if you like this series then you will also like Sara Donati's Into the Wilderness series. In fact if you have already read the Outlander series, you will find some familiar friends are in the wilderness as well. :-) Check that series out at Sara Laughs