“the act of leaving a book unread after buying it, typically piled up together with other such unread books”
This sounds so negative, doesn’t it? This aspect of book hoarding implies a dog-in-the-manger attitude: I can’t/won’t use it but, yah boo, you can’t either. It smacks of those billionaire recluses who buy artworks at auction only to hide them away from the public, keeping them as investments in a locked vault. But of course true book lovers don’t do that. Their intentions are good: to read the book at some point in the future and then, hopefully, share their thoughts about it with anyone who’s prepared to listen.
You want to know what this word is, don’t you? (Or perhaps you already do, and are patiently smiling because it’s taken me so long to cotton on to it.) Here it is: tsundoku.
Not to be confused with sudoku, of course. The elements of the Japanese word are “pile + up + read”. That doesn’t seem to me to imply that the books will remain unread. Wiktionary suggests a pun on tsundeoku, “to leave piled up”, but the original doku element strongly suggests that reading is involved. Eventually.
Tsundoku. My current favourite word. Those piles are slowly being reduced by the simple expedient of reading them. And then passing them on.