On her website, author Diana Gabaldon provides a chronology for all the books in her Outlander series. She specifies that her larger novels should be read in order, starting with "Outlander," while smaller novels and novellas are best read after "Voyager" but can also be read as standalone books.
Before listing the recommended chronology, Gabaldon separates the books in the Outlander series into three groups: The Big, Enormous Books, The Shorter, Less Describable Novels, and The Bulges. As of 2015, the first group of books includes all eight of her major novels, and Gabaldon specifically states that, for the best reader experience, these books should be read in chronological order, working from "Outlander" to "Written in My Own Heart's Blood."
The Shorter, Less Describable Novels are primarily historical mysteries, while The Bulges include various short novels and novellas. Most of The Bulges act as prequels and sequels, explore the lives of secondary characters or fill the time gaps in some of the larger novels. Many of these small works take place in one of the gaps in the novel "Voyager," so Gabaldon recommends these pieces be read after that novel. However, she adds that all of her shorter works are written so that they can be read alone, independent of the rest of the series.