I personally do not buy books, prefer to listen to audiobooks and therefore less technical subjects. For work I primarily read papers and learn about new "stuff" (languages, tools) with online docs and manuals. I also never did a technical certification.
It's a fact that the industry of technical books is in decline, and I'm sure there are opportunities to "refresh" it with new ideas and new styles of books. But before that, I was curious to understand what are people buying, are they actually consuming the content, what's the specific topic/use case.
The next one waiting on the book shelf is "Compilers" (Lam, Ullman).
Why did I buy them? I found them at the flea market for almost nothing, unopened (different occasions). They were impulse buys.
Recently I also re-read "C++20 - The Complete Guide" (Josuttis), just as a reminder. But it was a PDF.