While book 1 and 2 can be read as stand alone ones, I consider Split (book 3) and Afterglow (book 4) as one book in two parts. And they are definitely the best, and darkest, part of the story.
I like that Saki-sensei not prolonging this series and how she built the relationship between Munechika and--the not so typical uke--Shiiba with Shinozaki, Kaname, and later on Motoaki with them.
Even if on most yaoi novels/mangas you can almost always predict the outcomes, S is above the average. I don't regret reading the series at all. :)