This is the 559th most frequent Dutch word.
erbij
"Erbij" means "with it," "along with it," or "included."
Erbij here is used to indicate presence or inclusion of the books at the location I refer to.
Ik kwam net thuis en zag dat de nieuwe boeken al erbij waren.
I just came home and saw that the new books were already there.
Erbij here means adding something to an existing thing or situation, in this case adding ketchup to the sauce.
Doe de ketchup erbij als je de saus lekkerder wilt maken.
Add the ketchup if you want to make the sauce tastier.
Erbij here is used to emphasize wanting someone else included in the event or situation.