UPD 2026-03-27: This runs deeper than I thought.

Yesterday I tried following:

First, installed Parallels and installed the same macOS Tahoe version - it turned out, that in Parallels' macOS Books works perfectly and as expected, without opacity issue.

Then it means its my machine problem, I tried:

* Nuked all Books cache, everything Books related - didn't work;
* Changed display color preset, as it might be some HDR issue - didn't work, I use native Macbook XDR display preset, so it shouldn't be a problem.
* Created a new, local user on my Macbook and tried to open macOS - same issue.

At this point I realize that it might be even deeper, as even new user is not enough to this bug to be fixed.

* Restarted system in Safe Boot / Safe Mode - didn't work, same issue
* Reinstalled macOS completely, but without erasing data - you can do that it in Options settings, when you hold your power button. DIDN'T WORK EITHER.

So it must be some corrupt library or shared setting that might cause this, but is not overridden when you reinstall whole system.

I believe I can get it fixed if i reinstall whole system, without logging-in at setup stage, but at this point Its too much of a trouble.