Friendship is a very under-explored topic in fiction as a whole, I've noticed.

I get the impression it used to be a lot more popular -- in a lot of old adventure novels, for example, the main relationship in the story consists of the uneasy or otherwise loyalties between the male characters, while the love-interest is largely there as an abstract ideal for motivation.

Friendship between female characters isn't so common as a theme, partly I think because women tend to be seen as rivals for male interest rather than team-mates who depend on each other. (And I get the impression that female characters really don't go in for that intense loyalty bond -- unless, again, it's to the man she loves. A man will give up his life for his friend; a woman will give up her life for her lover.)