The Royal Historian of Oz is a collective title given to the writers of the books of official Oz history.
L. Frank Baum was the first Royal Historian of Oz. His first Oz book, The Wonderful Wizard of Oz was published by George M. Hill, and this was subsequently followed by 13 other Oz novels that were published by Reilly & Britton/Reilly & Lee. As Baum's series of Oz books progressed, the illusion created was that characters such as Dorothy Gale and Princess Ozma relayed their adventures in Oz to Baum themselves (usually by means of wireless). In this manner, the Land of Oz was presented to readers as an actual place, with a "history" that was chronicled by Mr. Baum.
After Baum passed away, Reilly & Lee published an additional 26 Oz books over the ensuing decades - 19 by Ruth Plumly Thompson, 3 by John R. Neill, 2 by Jack Snow, one by Rachel Cosgrove, and one each by Eloise Jarvis McGraw and Lauren Lynn McGraw. Reilly & Lee styled them as "Royal Historians of Oz" as well, and some fans still consider them to be just that.
A century after Baum began the Oz phenomenon, the L. Frank Baum Family Trust selected veteran author Sherwood Smith to continue the series of Oz books. The Trust commissioned four new novels from Smith; the first two, The Emerald Wand of Oz and Trouble Under Oz, were published in 2005 and 2006 respectively. Her third book Sky Pyrates over Oz was released in 2014 and concluded the "trilogy". The Smith novels are printed with a seal proclaiming that she has the "official license" to be Royal Historian of Oz.