This is a retelling of an ancient Greek story about a king who loved gold so much that when a god gave him a wish he made the wish that everything he touched turned to gold, which as we all know ended up having tragic consequences. In the original myth (and the original myth doesn't actually have him turning his daughter into gold, that actually came much later, around the 19th century) Midas found one of Dionysius' followers in a drunken stupor so he brought him into his palace, fed and clothed him, and then returned him to Dionysius, who as a reward gave him the golden touch (which is what Midas had asked for).
Anyway, the story ends in one of two ways. In both versions Midas discovered that his golden touch meant that he could not eat or drink, and in one of the versions he ends up dying of starvation. In another version, he prays to Dionysius who reverses the curse, but to do so Midas must wash in one of the rivers, which he does, and as a result of the power flowing into the water, many of the stones in the river bed turned into gold, which as it turns out is the case with this particular river, so it is believed that the story came about as a myth explaining why this particular river was rich in gold.
This particular book has quite a happy ending, but it does not mention Dionysius, instead having a fairy appear to grant Midas his wish. When I compare this particular story with the original I must admit that it is quite deficient (yes, I know, it is a children's book, but just because it is a children's book, does not mean that the author has to do away with a reasonable plot). My main gripe is that there is no reason that the fairy should approach Midas and offer him a wish. At least in the original version it was a reward for looking after one of Dionysius' servants.
The story has another interesting aspect to it in that our greatest dreams and our greatest wishes may not, in the end, be all that great. In this story (this particular book, not the myth in general) we have Midas who is incredibly wealthy, but he is not satisfied with the wealth that he currently has and he wants more, however in his desire to obtain more wealth he ends ups destroying himself and those around him. It is unclear in the original story whether he already had great wealth, but since he was a king, this probably goes without saying.
Therefore, the two concurrent themes in this story would be that the desire for more wealth can be a very destructive desire, but it also shows how even if we have much, that much is never enough and we end up always wanting more. It is the idea that we believe that wealth will secure us on this Earth, and as long as we have much wealth then we will not need to worry. However, there is always that fear that what we have may not actually be enough, so we will always go out to get some more, and in doing so we end up pushing other things aside. A person who, through their desire for wealth, neglects family, friends, and even their own health, so that even if they have the wealth, and their future is (so they believe) secure, in the end that future, even though it is secure, is not necessarily happy.