Complex sentence: The first book is poetry, the second book has the six plays she is largely known for (including "Dulcitius"), and the third book is more poetry and also the text from which historians have been able to deduce her approximate year of birth.
Split into simpler sentences: The first book is poetry, while the second book has the six plays she is largely known for, including "Dulcitius". The third book contains poetry and also the text from which historians have been able to deduce her approximate year of birth.