In the Kokoro series:
Vol.1 - Otokogokoro (A Man's Heart)
1 & 2. Otokogokoro & Otonagokoro
Salary man Saki Hiroyasu(33) passed out from fever almost before reaching home & been nurse back to health by a college student Shinomiya Jun(18) who was passing by..... When Saki realizes that Shinomiya hs been taking care of him for 2 day...not wanting to be indebted he offers money which Shinomiya rejects... Luckily, Saki's pretty creative when coming up with a way to express his gratitude that has them both wanting more...
3-4. Kiss Kiss Kiss
5. Nibukoi (Dull Love)
6. Hiiragi-senpai no Koi no Ohanashi
7. W Pure
Vol.2 - Koibitogokoro (Heart of Lover)
Vol.3 - Koigokoro (Heart of Love)
Vol.4 - Tokimekigokoro (Heart of Excitement)
The number of chapters comprising the main story in each book is as follows:
Otokogokoro (2 chapters),
Koibitogokoro (6 chapters + omake) entire book,
Koigokoro (2 chapters + omake), and
Tokimekigokoro (5 chapters + extras) entire book.
Kokoro (kanda neko)