The Illuminated Gospel of St John

That is indeed stunning. John is the most poetic of the Gospels. The King James translation isn't especially accurate, but it's powerful. It deserves a good illuminated edition.

I'd love a set of annotations of the inspirations for each illumination. Medieval illuminations are heavily coded and full of allusions that would go over my head.

Indeed. But, the text is so illuminated it borders on unreadable, like something straight out of Part 4 of Jonathan Livingston Seagull, where the ritual overtakes the flight itself.

I had heard KJV had some translation issues, but I'm surprised to hear that about John. I tried to learn Koine Greek, and a lot of the lessons started with John 1, so I always assumed it was one of the easier things to translate.

You could spend a whole semester just on John 1:1. The word "logos" is all over the place.

The KJV is an exceptionally good translation, and its flaws are particular and well-known. It was made at a time when all the most intelligent people of the world were doing Latin Greek and Hebrew, instead of say physics or javascript.

What I love about the KJV is that Queen Elizabeth had encouraged an entire nation of poets, and they were still around for James. It was cool to write poetry, and everybody did it. (She banged out a few decent verses herself, for that matter.)

Even today, KJV is the "voice of God". When you want to write a divine voice, you mimic KJV. "Thou shalt not..." just sounds more authoritative than "Don't", even though they mean the same thing. (I'd argue that "don't" better preserves the meaning, which was not meant to sound archaic. "Thou" sounded informal in the 17th century, but that's not how it sounds now.)

The flaws in KJV run deep; it's a very opinionated translation. Which is fine; every translation is opinionated. But there's a risk that people assume it's definitive because it's so powerful. There are some who literally claim that it is truly definitive -- even more so than the Hebrew and Greek originals. (This is of course insane, and that's the fault of horrific theology rather than the translation.)

Most of my reading is in Greek and Latin (no Hebrew), and I disagree with your evaluation. My evaluation remains as above.