This is the 44th most frequent Hebrew word.
בו
"In him" or "in it."
In this sentence, 'בו' refers to 'in him', indicating trust or faith in someone.
אני מאמין בו.
I believe in him.
Here, 'בו' is used to mean 'with him', suggesting accompaniment.
הבוקר יצאתי בו עזר הביתה.
This morning I left with him to the house.
In this context, 'בו' means 'in it', referring to the possession or containment of information within the book.