The important distinction: a novel can begin with a real mystery without claiming that its fictional answer is true. Good historical fiction shows where the surviving evidence ends.
Why unresolved history works so well
An unsolved cipher already contains the beginning of a mystery. So does a sculpture that vanished during a war. Readers have incomplete evidence and more than one possible explanation. History becomes a question to investigate rather than a list of dates to remember.
What should readers look for?
First, check that the historical mystery is supported by reliable sources. A popular legend may be part of the story, but it should be labelled as a legend. The fictional solution should not be passed off as a real discovery.
The history should also affect the plot. Old records need to matter. The setting should change what the characters can learn and what choices they make.
Real starting points in the Tianna books
The series begins with the Beale ciphers. The next book follows the wartime disappearance of an artwork attributed to Michelangelo. Book three draws on the divide between Atahualpa’s recorded ransom and the Llanganates legend.
The young characters and their modern investigations are fictional. The history guides on this site explain what is known and what remains uncertain.