Edit: it's Bliss by Lynsay Sands. Thanks @truthfndr !!!

The FMC is from the neighbouring estate and she keeps taking in peasants from the MMC's estate because he's so cruel to them. He is at war and all his instructions arrive by letter.

He returns and for some reason they have to marry. The FMC makes her breath stink to try and make him call off the wedding. They still get married. On the wedding night she covers herself with garlic (I think) and he orders a bath but puts flower shampoo in it and she's allergic to flowers. Pretty sure they don't actually do it that night.

Spoilers:

The big reveal is that the MMC isn't a total jerk, he's dyslexic and he has no idea what is going on. His half-brother has been writing all the letters and trying to ruin his legitimate brother. The FMC insists that he needs to learn how to read so this won't happen again and the book ends with them playing strip-reading-practice.