This is the 4587th most frequent Swedish word.
ag
"Ag" in Swedish means "reed" (a type of grass).
In this sentence, 'jag' means 'I', the personal subject pronoun used as the subject of the sentence.
Jag arbetar på skolan.
I work at the school.
Here, 'jag' again acts as the subject pronoun referring to the speaker expressing thought.
Jag tänker att det är en bra idé.
I think that it's a good idea.
This usage of 'jag' illustrates the personal pronoun referring to the action of purchasing a book performed by the speaker.