Possessive suffixes and plurals

[Phineas]

My Finnish text tells me that "there is no distinction between singular and plural possession,"
and gives several examples like "minun kirjani" for "my book" or "my books." A few pages
further on, I come across the sentence "Heidän lapsensa ovat minun serkkujani." The last
word seems to be a possessive suffix attached to the partitive plural. Is this correct? If so,
why do we need the plural if the "ni" is attached?

[007]

Minun kirjani = kirjani = my book
Minun kirjoja = kirjojani = my books

when it states there's no distinction between singular or plural possession, it means that -ni, -nsa etc. remains unchanged irrespective of the word's singularity/plurality e.g. kirja-ni, karjoja-ni. Similarily serkku-ni, serkkuja-ni etc.

[Phineas]

Minun kirjani = kirjani = my book
Minun kirjoja = kirjojani = my books

when it states there's no distinction between singular or plural possession, it means that -ni, -nsa etc. remains unchanged irrespective of the word's singularity/plurality e.g. kirja-ni, karjoja-ni. Similarily serkku-ni, serkkuja-ni etc.

Thank you. The text is confusing on this point. To quote it: Notice that there is no distinction between
singular and plural possession. Kirjani can mean both "my book" and "my books."

Is this incorrect?

[jahasjahas]

Nominative + the possessive suffix looks the same in the singular and in the plural:

Minun kirjani on hyvä.
Minun kirjani ovat hyviä.

The other cases look different, though:

minun kirjaani (partitive singular)
minun kirjojani (partitive plural)

minun kirjassani (inessive singular)
minun kirjoissani (inessive plural)

etc.

[Phineas]

Got it. Thank you.