their internal chronology, so that the next book published follows the previous book. How much these changes matter will vary from series to series (and reader to reader). For some, it may be minor—characters might get engaged, change jobs, etc., but it does not affect the main storyline. Examples of this type include Tony Hillerman's Jim Chee and Joe Leaphorn books. In other series, the changes are major and the books must be read in order to be fully enjoyed. Examples of this type include the Harry Potter series. There are some book series that are not really proper series, but more of a single work so large that it must be published over two or more books. Examples of this type include The Lord of the Rings volumes or the Dark Tower series by Stephen King. Some authors make it difficult to list their books in a numerical order when they do not release each work in its 'proper' order by the story's internal chronology. They might 'jump' back in time to early adventures of the characters, writing works that must be placed before or between previously published works. Thus, the books in a series are sometimes enumerated according to the internal chronology rather than in publication order, depending on the intended purpose for the list. Examples of this series include works from the Chronicles of Narnia, where the fifth book published, The Horse and His Boy, is actually set during the time of the first book, and the sixth book published, The Magician's Nephew is actually set long before the first book. This was done intentionally by C. S. Lewis, a scholar of medieval literature. Medieval literature did not always tell a story chronologically. Definitions There is no useful, formal demarcation between novel sequences and multi-part novels. Novels that are related may or may not fall into a clear sequence. It is also debatable whether a trilogy is long enough and whether its parts are discrete enough to qualify as a novel sequence. For example, the Barchester novels of Anthony Trollope are only loosely related, although they contain a recurring cast of characters; his political novels about the Pallisers have a tighter connection and dynamic. A strict definition might exclude both. History The novel sequence was a product of the nineteenth century, with James Fenimore Cooper's works appearing in the 1820s, and Anthony Trollope's Barchester books in the