This is a remix of various Ninomiya Hikaru's titles, including "Otoshigoro" that was never published in any previous collections.
List of titles and the books they were previously published in:
Sono toki, kanojo wa...? (Futari de asa made)
Otona no kankei (Koibito no jouken)
Midareru (Fukushuu no you ni)
Koibito no jouken (Koibito no jouken)
Himawari (Hatsukoi)
Nora (Fukushuu no you ni)
Otoshigoro
Engage (Hatsukoi)