In the Kokoro series:

Vol.1 - Otokogokoro (A Man's Heart)

1 & 2. Otokogokoro & Otonagokoro
Salary man Saki almost makes it home but passes out from fever just as college student Shinomiya passes by. When Saki comes to he realizes he's in a strange apartment and Shinomiya has been taking care of him for the past two days! Not wanting to be indebted, Saki tries to repay him as thanks but Shinomiya's having nothing of it. 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.