@boyd is there a 5.2? c6 seems to pick up right where c5 ended, so unless its like a side story or something, id just say its author error
*edit*
Raw scanner error. assuming these are released in parts, since the chapters themselves dont have a X.X numbering system on the title pages, the .X system is what the raw scanner uses, they probably assumed there would be another set and instead the author went straight to chapter 6 instead.