Truman Capote's novella differs significantly from the great movie that was based on it. I first saw the movie in my late adolescence, and it's the characteristics and insecurities of that time in life that the story reflects.
The two main characters are outsiders. They are Holly Golightly ('Traveling' her card says) and the nameless narrator. Unlike the movie, there's no romantic tie between the two. Their connection is an asexual friendship, which facilitates viewing the narrator, an aspiring writer, as being a gay writer (like the author). His homosexuality, in that day, establishes his outsider status, while Holly's is secured by the fact that she's from a dirt-poor background and works as a call-girl. Both seem to lack stable relationships.
These features enable Capote to capture that moment in life when one has left home but has yet to mold an adult identity. The main characters' identities are malleable and in a state of flux -- neither one is known by a real name, and while not literally homeless they inhabit merely temporary lodgings. Neither character has found a place in the world.
Capote captures a stage in life when social connections have a transitory feel, when the members of one's cohort are all on their way to some vaguely dreamed of future. While this stage has its moments of joie de vivre, it is also marked by friends and acquaintances who all too soon withdraw into their diverse futures.