I take your 2nd para in a way, the cost of a book being so cheap when I do read up on something I tend to buy 2, sometimes 3, books on the subject so where I get stuck on one the other will see me through.
Still, I am discomforted by your book-buying habits; a book unread is a valuable resource wasted :) But each to their own way.
There are lots of fantastic books out there, and only so much time to read them. Sometimes I buy some with the intention of reading them later. But reading books properly takes time, so it's inevitable that I fall behind.
Also, lots of the techincal monographs I buy have some overlapping contents with others I own.
I can buy a book for a single chapter. That's the only way to get that chapter, so it justifies the purchase to me, even if the subjects covered overlap with other books I own.
“Unread Books Are More Valuable to Our Lives than Read Ones”:
https://www.brainpickings.org/2015/03/24/umberto-eco-antilib...
This is true.
However, I find that I tend to buy much more than I read. Not just because I tend to hoarde these things, but also because I try to seek out sources that click with me before I work through them.