There are three series, but all three tell a single story. First is Nana to Kaoru, then NtK - Black Label, then this one (Last Year). Read them in that order.
There's no "stupid cuck bait" at all, everything is really well thought out and makes sense in the context of the story.
I just finished reading the whole story once and for all, and it was definitely worth it! I think it goes too far in some ways in some respects, but overall it's great and has some really interesting insight into psychology, relationships, etc.