This is the 1195th most frequent Spanish word.
relacionado
Related.
Here, 'relacionado' is used as an adjective to indicate a connection or association of the book with the topic of ancient history.
Este libro está relacionado con la historia antigua.
This book is related to ancient history.
In this sentence, 'relacionado' describes mutual connection or relationship among the incidents.
Los tres incidentes están relacionados entre sí.
The three incidents are related to each other.
'Relacionado' in this case refers to movies that share a common theme or connection with environmental topics.