Kondo's guiding principle, laid out in her previous book, is that you shouldn't keep items that fail to spark joy when you consider them (unless they are useful things like, perhaps, your house keys or your internet router). Spark Joy takes us in deeper, using cute illustrations to show us how to properly store the things that we decide are worth keeping around.
Kondo has written helpful sections on organizing books, papers (first tip: get rid of them!), komono (e.g. electronics, medicines, kitchen supplies, skincare products), and sentimental items. She then goes on to guide us towards making a home that truly sparks joy; living with other people whose habits may be different from our own; and using tidying as a way to clear our thoughts.
Could be a good first read for 2016!