Zenbu Hatsukoi no Sei da
ぜんぶ初恋のせいだ · ぜんぶ はつこい の せい だ
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Rei, a university librarian, is on the hunt for a marriage partner! In order to reassure her grandparents who raised her, she is looking for a suitable husband based on "attributes" alone, hoping to settle down as a “married couple." Then one day, Rei suddenly receives a public confession from Suwa, a college boy. Suwa is a transfer student with a straightforward personality, but of course he is much too young and totally out of the question! But he won't give up so easily…? (Source: Shogakukan, translated)
Source: AniList
External databases: AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections