This is the 58th most frequent Japanese word.
について
"について" means "about" or "regarding."
In this sentence, "について" is used to define the topic of the book—space, indicating that the book is centered around or discusses this subject.
この本は宇宙について書かれています。
This book is written about space.
In this example, "について" introduces the topic of discussion—environmental issues—signifying the subject matter being considered.
環境問題について議論しました。
We discussed about environmental issues.
Here, "について" specifies the topic of the question, which is his opinion, showing the context or scope of the inquiry.