"Knew like a book" is a common idiom used to express a deep familiarity with something or someone. However, there are several synonyms that can be used to convey the same meaning. For instance, one could say they "knew it inside out," "had it down pat," "had it memorized," "were intimately familiar," "were well-acquainted," or "were fully versed." Each of these phrases implies a comprehensive understanding of the object or subject at hand, suggesting someone who has spent a considerable amount of time studying or observing it. Using these alternatives can add variety and nuance to one's writing or conversation, making for a more expressive and dynamic communication.