The "Outlander" series of book can be read as stand-alone books and do not have to be read in sequence. While the books have the same central characters, each book varies in theme.
The author, Diana Gabaldon, has stated that although the books have been written to be independent of each other, she personally recommends reading them in the order they have been published. The spin off series "Lord John" can be read independently from the "Outlander" series but also as part of it after the third book, "Voyager."
A graphic novel set in the "Outlander" universe, titled "The Exile," covers material from the first book but from viewpoint different to those of the main character, Claire Beauchamp Randall. A companion book for the series was published in 1999.