The Other World's Books Depend on the Bean Counter is a fantasy manga series based on a light novel series. It's basically m/m isekai. I bought my copy of this first volume brand new.
Review:
Kondou Seiichirou is an overworked accountant who's heading home one evening when he hears a girl crying out for help. He rushes to her, only to get pulled along with her into a new world where she's expected and wanted and he's...an unexpected extra. The girl, Shiraishi Yua, is named the next Holy Maiden, a girl destined to save the kingdom from a deadly miasma. Kondou is asked what he'd like to do, and he requests an accounting job.
It doesn't take long for Kondou to start digging in places he isn't wanted, attracting some powerful enemies. He notices something fishy going on in the kingdom's accounting books and makes it his personal mission to clean things up and ensure that the country is financially stable enough to weather the hardships he's sure the miasma will soon cause. In the process, he overworks himself to the same degree he was overworked back in his own world, and tries to make up for it with "nutritional tonics." Thankfully, at least one of the people keeping a close eye on him is willing to help him out when he suddenly ends up in trouble.