Here's why:
* Metaphor: A metaphor directly compares two unlike things without using "like" or "as". It states that one thing *is* the other. In this case, the heart is being compared to a book, implying that one's emotions and feelings are easily understood.
* Simile: A simile compares two unlike things using "like" or "as". An example using a simile would be "His heart was *like* an open book."
Since the phrase "heart is an open book" directly states the heart *is* a book, it's a metaphor.