Arghhh, I lost the track of these lovers. There are so many mangas about them , can anyone...

Part 1: Asobikata
Part 2: Amaekata
Part 3: Nakasekata
Part 4: Itoshikata
Part 5: Afurekata

Spin off 1: Fukigen Kareshi no Nadamekata (Jaguar brother side-story)
Spin off 2: Nekoka Kareshi No Ayashikata (Shingo makes appearance)
Spin off 3: Nekoka Danshi No Shitsukekata (Same universe, different characters)

Hope this helps! ╮(￣▽￣)╭

Right! They should just have all the books in one place

I made a list that i myself use when I re-read this awesome series. Feel free to use it too! They're all in order here:
http://www.mangago.me/home/people/4365/home/

oops, lol, here:
http://www.mangago.me/home/mangalist/39827/

Nice (▰˘◡˘▰)
You missed Kagami's appearance in Spin off 3, he is Shiou's co-worker (Kagami was using catnip and Shiou was affected).
Also Shiou has appeared in Kuroneko series, but please don't ask me which volume ε=ε=(ノ≧∇≦)ノ (when Kagami was mad and wanted to do a murder scene, when they said both actors were going to act in the same movie(?) and when Shiou was thinking about Shingo's and Kagami's encounters in their workplace).
Ah, and characters from Spin off 2, appeared in Spin off 3, because MC from both are in the same school.