KnJ: Houkago is a collection of 1 offs along the subject matter of the first series.
It's good stuff, but not quite the same as the source material. There's good bits that weave in and out of the main story, and a final volume where Rin talks to Mimi and Kuro, her best friends, about her meeting with her teacher as "two adults would meet", but all in all don't feel like you must hunt these down if you enjoyed KnJ.
Knowing that none of it was really relevant to the story took some mood away, but if you must have more KnJ, absorb these like everything else!
