This is more of a prequel for the most part than a continuation of volume 1. The first half (or more?) goes back to when Harto meets Matthew on the "outside" and how they begin their "training." It's still a fun, smutty read, but wasn't as good as Volume 1 in my opinion. 

Maybe it's just me and not being a manga reader, but I had a hard time remember who was who and by the second half when it was sort of "epilogues" of volume 1 matches, I was more confused than anything. If you can find it and really want more, it's not bad, but I wouldn't say you're missing too much if you just stick to Volume 1.