History novels are useful for learning historical perspective, but they are not as trustworthy as original source sources.
As you might expect, you can't write history without sources since you'd be making stuff up (which is good in historical fiction, but rather problematic when it comes to serious history). Primary and secondary sources are often classified into two groups. Encyclopedias are considered primary sources because they collect information from many different authors at various times. Newspaper articles, books, and other forms of literature are considered secondary sources because they're based on original research or interviews with people who were alive during important events in history.
Nowadays, everyone knows that Winston Churchill was one of the most important political leaders in the world during the mid-20th century. But did you know he also had a major role in world war I? Or that he developed a taste for Scotch while serving as British prime minister? All true! He ordered all whiskey imported into the UK to be placed in a single location where officials could check it for quality control. When Churchill's personal stock ran out, he had another batch sent over from Scotland. That's how famous Scotch drinkers the Churchill family was back then.
In addition to being author, politician, soldier, and painter, Churchill was also a prolific writer himself. His letters, speeches, and other writings have been used by historians to understand what was going on in the world during the years he lived it.
Novels about history are often called "fiction," but that term fails to convey how much of what we know about past events is learned from novels. Novelists draw on facts and create fictional characters to tell us about people who have lived before our eyes. Fiction allows us to gain new perspectives on old events as well as imagine different possibilities than what really happened.
The more widely read a novel is, the more information we can glean from it about history. For example, Henry V—one of the most famous kings in English history—has been described by many historians as being a short-tempered, misogynistic warlord who waged cruel campaigns against France's enemies. However, this image of him is based largely on descriptions in Shakespeare's play King Henry V. As we know from reading about his life, Henry V was not only a great warrior but also responsible for some important social reforms in England. He ended the War with France while still in his twenties (in 1453), thereby establishing himself as one of the greatest leaders in medieval Europe.
We can learn so much from reading about other people's lives because fiction is able to give us views of history that would otherwise be inaccessible.
Non-written historical sources should not be utilized to write history. Historiography's topic is history itself. Historiography is the study of how history is written and how our knowledge of history evolves through time. History books contain narratives about important people and events in the past, but they are not considered primary sources because they weren't recorded immediately after the fact.
Written records exist for anyone who was rich or powerful enough to hire historians to record their lives for them. These records include letters, diaries, official documents (such as treaties), and memoirs. Non-primary sources can give us insights into how history was experienced by ordinary people, but they cannot replace primary sources.
Primary sources are documents that have been directly copied from other documents, usually earlier versions of them. For example, a letter written by an ancestor of mine called William Henry Harrison is a primary source because it was copied from a copy of the original letter he wrote years before when he was elected president under the name William Henry Harrison. Secondary sources include works of history that were not written by people who lived during the period being studied; rather, they are based on primary sources such as letters, journals, and official documents.
Tertiary sources include anything else you might want to use as evidence for your argument.