Actually it's:

The American Heritage=AE Book of English Usage.
A Practical and Authoritative Guide to Contemporary English. 1996.


3=2E Word Choice: New Uses, Common Confusion, and Constraints


=A7 78. could care less / couldn't care less

I could care less! you might say sometime in disgust. You might just as
easily have said I couldn't care less and meant the same thing! How
can this be? When taken literally, the phrase I could care less means
"I care more than I might," rather than "I don't care at
all." But the beauty of sarcasm is that it can turn meanings on their
head, thus allowing could care less to work as an equivalent for
couldn't care less. Because of its sarcasm, could care less is more
informal than its negative counterpart and may be open to
misinterpretation when used in writing. 1
The phrases cannot but and can but present a similar case of a
positive and a negative meaning the same thing. For more on this, see
cannot under Grammar.

Source: http://www.bartleby.com/64/C003/078.html

Again, out of many times, sarcasm swoops over the malformed bald head
of Lloyd Davies. Taken literally, if one could care less, then one must
care at least a little, which is obviously the opposite of what is
meant. It is so clearly logical nonsense that to condemn it for being
so (as some commentators have done) misses the point. The intent is
obviously sarcastic-the speaker is really saying, "As if there was
something in the world that I care less about".