Sorry for the late reply! Actually, I just bought this guide from Amazon: https://www.amazon.com/dp/0691139261/ref=cm_sw_r_cp_apa_i_WBQfBbDJ3S967 It's a Princeton field guide that covers East Asia including Korea. Not sure if you could find it while you're over there though... I've never used iNaturalist before, is there an app or something?
I live in the outskirts of Gangnam, so in Seoul :) Heading back before the end of the month.
Most guides cover northern Asia generally. The Princeton guide, pretty good, too big to carry as a field guide IMHO. There's a photographic guide that's smaller. Surprised that a country that attracts many Western tourists lacks a good English solution. Hoping to find something there in Japanese but still useful.
Birds of East Asia from Princeton is probably a good one: https://www.amazon.com/Birds-East-Asia-Taiwan-Princeton/dp/0691139261