The Official Non-Fan Fiction
Holy shit I found the books
There used to be books. The show had books. Real books. Not many books - there were only like... three, maybe? But five planned in total? (I've had to update this twice already lol there were four published). Not all of them were released, proving Saban has always hated us. The ones that got published were just retellings of the first couple of episodes - but interestingly enough, they set them up as choose-your-adventures. Why? No idea, but I think it's inspired. The framing is that you're a seer of the future, and you get to navigate the Mystic Knights through their fate. I have to believe that this is why Tir Na Nog is so aggressively spying on everyone: so they can publish this stuff for their own entertainment.
I haven't actually read them yet. I did flip through one, and as a personally fantastic discovery to have made already...
Shit's heatin' up on the ol' Fae!Angus front, ehhhhhhhhh? >:3
Anyway, here are the links! You'll have to make an account on the Internet Archive first (which is free), and then click 'Borrow' at the top to see it. They're treating it like a library. Cool! Have fun. :D
1. The Legend of the Ancient Scroll
A Legend of brave heroes and great warriors...
"Rohan was an orphan, a lowly apprentice to the King's magical adviser Cathbad... until a fragment of an ancient scroll revealed his true identity. Now, along with his companions Angus, Ivar, and Princess Deirdre, Rohan must fulfill his destiny - become a Mystic Knight and save the Kingdom of Kells!
"You, dear reader, must use the powers within you to help Rohan. Guide him and his friends to the mystical world of Tir na Nog so they may earn magical weapons by proving their honor [sic >:3]. But make your choices wisely... for if they should fail, Kells will be destroyed by Evil Queen Maeve and the vicious creature under her command!"

2. Fire Within, Air Above!
Chants of powerful protection...
"The Mystic Knights have earned their magical weapons. Now another task awaits them - to win their mystical armor [sic - u damn americans and ur spellings]. But each set is guarded by a dreadful Evil Sentinel... who must be defeated in order to call on the elemental power of the armor!
"Rohan and Princess Deirdre must make the first attempts. But they cannot succeed without you, dear reader. By making choices to guide them, you can ensure that the Mystic Knights act as a team. For without loyalty, the heroes will be unable to gain their armor... and the Kingdom of Kells will fall to the dark forces of the Evil Queen Maeve!"
3. Water Around, Earth Below!
Beware of illusions...
"Now it is Ivar and Angus's turn to earn their mystical armor. In order to do so, Ivar must defeat Forgal Mac Roig, the Sea Serpent, and Angus must overcome Bane Morfan, the Rock Wolf. These fearsome Evil Sentinels will not give up the prized armor without a perilous battle!
"But Ivar and Angus have you on their side, dear reader. Choose their paths wisely, and the Mystic Knights will seek truth, honor, and loyalty - and will defend Kells despite the pitfalls of illusion. But if you give in to fear, the Mystic Knights will be easy prey for the Giant Cyclops and the Evil Bull of Temra!"
4. The Taming of Pyre
Time to fight fire...
"The time has come for our heroes, Rohan, Angus, Ivar, and Deirdre, to tame Pyre, the Fire Dragon of Dare. The four friends are confident that they won't have much trouble, thanks to their Mystic weapons and armor.
"But when they come face-to-face with the ferocious dragon, the Mystic Knights make a terrifying discovery. Their weapons and armor have no effect on Pyre! Things look bleak for the Mystic Knights...
"Only you can help them, mighty reader. Choose wisely, and the Mystic Knights will tame the Fire Dragon. But the slightest misstep could find the four warriors lost in a Pixie Field or locked in battle with the Evil Sentinels!"