He was an interesting guy. The coolest thing about him, from a writer's point of view, is that he probably didn't write quite a lot of books that are said to be his.
The same thing has happened to me, though unlike Byrhtferth I haven't had any major works of philosophy and science attributed to me, but only a short work called Shake The Maracas.
It's not the same, you know.
Anyway, Byrhtferth was a scientist (his Enchiridion was the best science text book of his time) as well as a historian (of saints, largely) and he believed that everything in the universe was linked together in a great pattern of numbers. That's everything, from the stars (an old idea) to the letters of the alphabet.
As far as language is concerned, Byrhtferth was the first person to write down anything in a manuscript in Ogham script. (Ogham itself is older much than Byrhtferth, but his manuscript is the earliest example we have that's not been carved.)
Byrhtferth had a great interest in alphabets (and pretty much everything else, bless him) and he almost certainly understood what he'd written down in Ogham.
The Ogham script is the telegraph-post stuff in the central circle.
Byrhtferth's work on numbers, which claims to delve into the secrets of the universe, is being used a lot by modern-day magicians, so Byrhtferth's Ogham script is of huge interest.
And what does it say?
No one's got a clue. Byrhtferth never bothered to write down an explanation.
Thing Not To Do Today: write down something in code and then not explain it to anyone.
The name Byrhtferth comes from the Old English byrht, bright, and ferhþ, mind. This was either a huge coincidence, or a pen name.