The manuscripts, books, and gospels, that we have seen from the Celts and Anglo-Saxxons are intricately and beautifully decorated. They are obviously not that easy to read, and honestly remind me a little of an adult Christian coloring book.
In the Classical and Byzantine era, religious art was intended to invoke emotions and inspire people to repent. The message was about the salvation of mankind, and art intended to speak of this miracle. One thing that you definitely can see has influenced the art of the Early Medieval times is the gold, colors, and intricately decorated pages and artwork. If we look at the Illuminated Manuscripts from the Mid Byzantine era, we can see that the thick decorated borders and pastel looking colors are very similar to the manuscripts of the Early Medieval time.
I think the form and function that the Byzantine art heavily influenced on Early Medieval art is that there needs to be an underlying message for the audience to understand. Not everyone could read during this time, so a lot of communications where through images if not orally. The message was of reflection on salvation.