No rule for this one
No ending or word group predicts the article of Buch. Learn it as a pair and say it out loud: das Buch.
das Buch
It's das Buch. Buch is neuter, plural die Bücher.
Das Buch ist interessant.
The book is interesting.
No ending or word group predicts the article of Buch. Learn it as a pair and say it out loud: das Buch.