I used it to read some webnovels a few months ago. It's much better than other machine translators (like google or deepl) but it's a bit tedious to use because of the character limit.

Deepl might do a decent job and it supports pdfs and other document formats. We use it a lot for translating business documents and contracts. Mainly between German and English. But they support Japanese too.

Sceptical that an LLM would produce good literary translations - that requires a nuanced understanding of the author's rhythm and style. Maybe if the model was trained only with writing from that one particular author.

Negociating with a Japanese publishing house is probably 3 times the effort of translating the text itself. I'm interested in the book myself and I can read some Japanese (and know people more proficient than me who could translate it), but that's a no go because of intellectual property.