This is a spinoff from the main Saint's Magic Power is Omnipotent manga series, and I certainly wouldn't recommend reading it until you've read a good few volumes of the parent series. For those of us who already have though, this is a fun and cosy addition that's well worth a read, although I wouldn't consider it essential (not least because the original light novel does not have a spinoff).

It is unfortunately the case that a lot of this volume retells events from the main series, albeit from Aira's point of view and with new scenes added in between. I think it was basically inevitable for the first volume as Aira's 'origin' story has already been told, but I really hope that from the second volume onward the ratio leans more heavily toward new content (albeit I know certain events will still inevitably need to be featured). 

Overall I had a really good time reading this but I do consider it a volume aimed mainly at dedicated fans of TSMPIO (of which I am definitely one).