Most authors are keen for their books to be on your shelf. Presumably Marie Kondo is no exception, an author who has made a name for herself with instruction manuals for household management. She is the queen of declutter, which means she can assert her authority to tell you that the right number of books to keep in the home is 30. That's not a minimum figure, that's the maximum.
Perhaps she gets away with saying 30 because she has good delivery. Presumably among the 30 books still remaining on your shelf post-declutter are those written by Kondo. You never know when you might need to reach for your downsize bible, the way things stack up on a weekly basis. Which of your 30 books you retain is up to you. The main idea is to simplify your living space, and therefore your life, in every particular.
Jeanette Winterson somewhere years ago pronounced that a person's library should be about 100 books. She is an author of persuasive directness who seems to have grown up in a home where books were anathema. Or not even anything so reactive. Books were not to be had in the environment. Such a puritanical childhood could leave a person wondering if books had any value at all, so even to achieve 100 under one roof could be judged a breakthrough. It is like opening the floodgates, more especially if some of the books are the wrong kind of book.
When we consider how many people do not have 100 books it can seem like an adventurous figure, but if your library exceeds triple figures as a norm, it suddenly looks quite tight. I guess Winterson wanted something manageable, a resource where all her favourite writings, influences and references were within easy reach. Don't we all? While 100 books should be the basis for a classical education, or just an education, and we would expect Winterson's 100 to be quality reading, her pronouncement implies there is such a thing as enough, even in self-education.
Derision is the general response of booklovers to Kondo's magical 30. Asks the modest meme: Is that 30 books in every room? Does she mean the 30 books on my coffee table or the 30 against the sofa? The 30 on my nightstand? Readers with 3000 books see no need to stop. What's life for if not acquiring more books than you will ever read?