Many history books are written for the general public rather than scholars. A 'popular' history book isn't always inaccurate - but it may often take liberties in the name of entertainment. For example, popular history books may make up details of a conversation, or speculate on what is likely without providing solid sources.
So how do you know if it's scholarly? There are two main clues:
Ask these questions to get a sense for whether your source is reliable:
Think about these questions when evaluating primary sources: