Mofumofu to Mukumuku to Isekai Hyouryuu Seikatsu is a Japanese novel of 1,189,032 characters, with 16,578 unique words and 2,178 different kanji. Its difficulty rating on Jiten is average, 2.9 out of 5. The full vocabulary list ranks all 16,578 words by how often they appear, and you can study them on Jiten or download them as an Anki deck.
Here are some of the words that appear most often in this media, as well as a sample of its rarer vocabulary.