The Dragons Of Inheritance

Just to be clear, I give this a generous rating on purpose, because although each coverpage illustrates each dragon's appearance (Including the new Shruiken poster art in Inhertiance Delux), and the various descriptions in the books, MUCH of their appearance is left up to the imagination. So there really is no going wrong, simply because there is no incorrect way to do art.
(I'd rather start with the flaws, because then we end on a good note) What caught my eye initially was that the scales on the side of Saphira's cheek are far too .. "Wispy" in my opinion. But I see the majesty you are trying to capture by doing that.
Secondly, Shruikan's eye. The area around the eye looks cracked, and it's peculiar and eerie. Also, I wonder what you are doing with the area around and above. What could be two mountains, or alternatively, two halfs of his face, is displaced slightly. If you were going for the Shruikan's face option, the area all around the eye and face lack detail, making it stand-offish. But nonetheless, artistically done, not a major flaw.
Third, not so much a flaw, but just an interesting interpretation of Glaedr. He is mentioned VERY often in Eldest and Brisingr as being immensely bulky, thunderous in his personality. You made him look far more young and lean than I think Paolini intended, as he was supposed to contrast Saphira. He is also far smaller than he is mentioned in the book.
Finally, On the left side (Not all that important) I find the fire a little repetitive. It distracts the attention from the dragons.

On to the good stuff! The personae you've put into the four main dragons (Not counting Shruikan) Is absolutely spot on in my opinion. Glaedr is confident, with the outspread wings and locked knees. Thorn is distraught looking, like he's always in pain, due to Galbatorix's control. He's defensive, and he really has that "Hatchling in a grown dragon's body" look. Saphira looks determined, proud, (a little bit manly) and free. You captured, quite literally, what Paolini would call often "One big brilliant sapphire eye". (Also, the colouring of her scales tips the balance between someone who's read the book, and someone who knows of the book. You incorperated all the pale blues and purples, which is awesome!) Firnen's got that bounding hatchling confidence, despite what little we know of him, He looks firey. I like that.

So perhaps your stylistic devices aren't completely original, it's still unbelievably unique, I can't see what more anyone could ask for. Good work.