In the last chapter 158 of manga MC was about to train his spirit power by manual from landlady's collection, you kinda can say that this is a backlash from the future messup xD
nice, there are also two other yuragisou doujins by the same author on dlsite: https://www.dlsite.com/maniax/work/=/product_id/RJ216514.html https://www.dlsite.com/maniax/work/=/product_id/RJ251978.html