I usually just ask Jess on questions involving the comic I figure she would know.
I hope I would know!
Anyhow- as to the markings, most of the design is taken from both tribal ideas of tattoos. The Marks, for instance, on James's face, are like an archaic power grid-the more that light up, the higher he is. It also means that when their's an odd number of marks-like, for instance, when James's marks first appeared- He was a tad unbalanced. It you go back to the section where his Godself (the whitehair one from the dream sequence of Double Lives) has two sets of markings on either cheek, each with an even number of marks.
Jessica, while she has no visible marks at the moment, can increase her power flow by decorating her body with spells-making herself a sort of living tapestry. This is why she was wearing a long outfit when at the washington party- she had slapped wards all over herself. Another comic that used this idea (in sorts) was Orion. Ah! My Goddess! uses markings as well, but they don't ever disappear. James's don't unlock as theirs do, and if he really tried, and sat down and actually read all the stuff that Jess found on him, he could have them form in different shapes-the ones he has now are the same ones he had when he actually was a god, before the world was remade. (Whoops, did I give that away?) He has exactly half the markings his father has, and his da can make his into the wards that actually make up the markings-making him look like a wizard would. The marks are actually hundreds upon hundreds lines of god-spells. The shape they take (but not the number) is mostly up to the will of the marked.
Confused? I am.
And the dragon, as well as others, appear again soon. Particularly after Timora (Timmy!) explains about the Immortals, and why they want Jessica.