volume 6 is divided into two volumes, 上 and 下. I haven’t read them yet myself, but the first volume has the prologue, chapter 1 and the first half of chapter 2, while the second has the second half of chapter 2, chapter 3, the epilogue, the extra file and the afterword, according to their respective tables of contents. (o・ω・o)
hope that clears things up!
the different covers are the older versions of yakumo (how they were when they were first published in hardback) while I translate from the newer and revised paperback versions. the earlier hardback versions were published by bungeisha, and then kadokawa took up the series and published it in paperback (and are also publishing the hardback ones now). they’re basically the same, but here are some changes:
hardback volumes are still being released first - volume 9 is out in hardback but not paperback yet, if I recall correctly. there are also special volumes like secret files: kizuna (a story from before haruka and yakumo met) and another files: itsuwari no ki (originally a stage play). I hope that clears things up for you (and anyone else who was curious) ♫