I’ve been reading quite a number of young adult series novels of late. Partly, for Cereal Readers website content; partly to preview material for my own YAer; and partly (mostly) because I enjoy the genre. I was somewhat perplexed when I read the phrase—issued with disdain and defiance from an adolescent protagonist—"I could care less!".
I read this twice; three times. Was this the oft-encountered e-book typo? Did the character care more, rather than less? Was the author just confused about the saying?
I read on, and the dialog slipped from my mind, but I soon encountered it again…and again. Let me clarify that I encountered this phrase in YA novels by American writers. Not every writer 'could care less'—some 'couldn’t care less'—but still, it was obviously not an error as I first thought, but rather a cultural difference in the saying. I have spent almost seven years of my life in the U.S. (including a year as a high school student) so I was amazed this difference hadn’t come to my attention before. Perhaps when issued in verbal form, my brain had just filled in what it thought was missing or meant? Regardless, what bothered me most was the lack of logic of it: if you 'could care less', you could feasibly care a LOT, which was obviously not what the characters in these novels wanted to communicate.
Hmm, so what was this about? I couldn’t let it go. I really cared—a lot (just to be clear). A little investigation was required, and such nuances are the life-blood of the Internet.
In brief, here’s what I found out. 'Could care less' is unique to America, and whether it developed from a sarcastic interpretation, careless pronunciation, or some other factor—or indeed, combination of factors—is unclear.  It is not a new issue, by any means, and has
certainly elicited controversy and heated discussion. As recently
as March 2013, the furore around that phrase 'could care less'—uttered
by songstress, Taylor Swift—was the motivation behind this article in the Chicago Tribune.
Assuming the sarcastic meaning, which seems to be well-supported, I’ve been walking around the house uttering, ‘I could care less’; ‘I couldn’t care less’, in varying tones. (My husband is looking at me like I should be committed.) I don’t know...it always seems to be something one says in defiance and anger; the gauntlet thrown down. Is the sarcasm really there? Perhaps I need to actually hear it said by an American (Taylor Swift?)...but I'll admit, it's hard for me to get away from the logic of 'couldn’t care less'.
'Could care less' is certainly not the form used in all American YA novels I’ve read, so either it’s somewhat regional, or there are writers attempting to be less parochial. Or as Maeve Maddox suggests, “Although the newer form of the expression meaning “not to care at all” has been widely-used for some time, many people still regard it as an uneducated error.” 
Michael Quinion also discusses the difference with interest and includes a linguistic point of view, but concludes that “it is still regarded as slangy, and also has some social class stigma attached. And because it is hard to be sarcastic in writing, it loses its force when put on paper…”. Perhaps, as Grammar Girl suggests, when in doubt, go with 'couldn’t care less'.
As an American YA writer, you might not be aware that this form of the saying is colloquial to America; that it does—in the midst of riveting and emotional dialog —mentally trip the non-American reader; something of a foot whipped out as you're walking past. Perhaps you’d like to consider the wider market and the dissolving international publishing boundaries; but then again, perhaps you couldn’t care less.