This is the 1340th most frequent Polish word.
pierwszą
"Pierwszą" means "the first" (feminine, accusative or instrumental case).
The word 'pierwszą' here is used as an ordinal adjective, agreeing in gender, case, and number with the noun 'osobą'.
Zosia była pierwszą osobą, która do mnie przyszła.
Zosia was the first person who came to me.
The word 'pierwszą' is an ordinal adjective describing the noun 'książkę', in accusative singular form matching its direct object role in the sentence.
Kupiłem pierwszą książkę z tej serii.
I bought the first book from this series.
Here, 'pierwszą' modifies the noun 'miłość', functioning as an ordinal adjective in the instrumental case, agreeing with the verb it relates to.