Because books are neutral.

The displacement of possessive pronouns is determined by the nature of nouns added after 1 and the case after verbs.

Positive and negative neutral complex number

The first lattice (N) mein mein meine meine

The second grid (G) meines meines meiner meiner

The third box (D) My mine, my mine, my mine, my mine.

The fourth case (a) meinen meinen meine

For example, ich mag dein buch, followed by mag is the fourth cell, buch is neutral, and dein buch is in the corresponding table.

After Ich helfe meiner Schwester, helfen is the third box, which means "Sister die Schwester", which is negative, so it corresponds to meiner Schwester.

If the typesetting is not arranged properly, just have a look. Sorry.