If I remember correctly its a full one vol manga, the story is about a girl in library or literary club, who loves her older sisters new husband (so those three now live together, the older sister have been with him and broken up with him multiple times before), then her kouhai (who is short and in same club) confesses to her. Those two gradually gets closer, hmm another thing to add later some books gets missing that the kouhai has rented so they think someone stalks him.

Sorry for poor grammar.

Osuteki Darling , I think that's the manga you're looking for since it fulfills all of your requirements. =3

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"Beware How You Take Hope From Another Man."

hmm thought it was complete, but it was correct manga, thanks