This is in the section "conjunctions," but I think I'm correct that 'için' is in this case a postposition. You'll recall from that section that some postpositions, such as 'hakkında' or 'gibi', generally take nominative objects, but require genitive object pronouns.
Thus, "öğrenciler" is in the nominative because it's a noun, and "sizin" is in the genitive because it's a pronoun.
To my knowledge it would be "kitabı" only, if "the book" was an accusative object. You don't use the suffixed form in nominative (for the subject). Same with speaking generally about books. Singular form is used for general (plural) objects, not subjects (see AlexinNotTurkey's answer to armanduuu's question above).