I also found out that my colleague was right. People do like to read a series of books on a similar theme, especially when that theme is historical. Although I love writing historical fiction, I've never been a great fan of reading it. Until I read Hilary Mantel's Wolf Hall. What a tremendous book. I was thrilled when I knew she was writing a sequel to it and bought a copy of Bring Up the Bodies as soon as it was released. And there is still another one to come before she finishes the story of Thomas Cromwell. I await its release with bated breath.
There are other historical novelists who have used the series to great effect. The well-known writer Anna Belfrage has two historical series on the go. The Graham Saga, a fascinating series of books set in the 17th century, but with a difference. A young woman has travelled through time back from the 21st century to four hundred years earlier. A prolific writer, Belfrage has just published the ninth book in the series.
She has also created an exciting series set in the 14th century about a minor knight and his family, called The King's Greatest Enemy which runs into three books at the moment but with more to come, I'm sure.
Readers like to follow the adventures of the same characters; they come to know them and either hate or like them, but the familiarity is comforting and they become invested in the outcome for these fictitious people.
Anne Mary Yard is another historical novelist who writes her books as a series based on one particular family; in this case it is the Du Lacs and it is set in Saxon Britain. The Du Lac Series runs to three books at the moment but the door is open for more adventures with the same group of characters.
As for my own al-Andalus books, they're not going to become a long-running series because the backbone to the books is the Omayyad dynasty and in Book 3, The Ring of Flames, the reader sees how the dynasty comes to an end. Of course I could continue with my hero and his family and follow his exploits for the next twenty years but I think it's time for me and them, to move on to different pastures. Maybe another historical series, set in a different time period.