Never heard of this "myth"-series. Checked it up on Amazon, and does not seem to attract my interest. (Sorry OP).
Diskworld: Well, I read one book a long time ago, but it was then the next book after the HitchHiker. That rendered Diskworld, in my opinion, a "copycat" of HTTG.
After I made a thread about that here a while ago, I bought the 5 first Diskworld books to my Kindle and startd reading.
Well, still I'm too HTTG-biased.
So for me there is only one Guide