I know this is about the 4.0 beta and I’m still running the latest 3.x. But I’m now on iPadOs 26 and windowing seems to be working fine, in fact better than under iPadOS 18 cause pagination in the reader doesn’t seem to be breaking anymore when going to other apps and back.
Resizing is also working as expected. You can’t have multiple instances, but hey, apple’s own Books app doesn’t allow that yet either. Other than that the only thing that‘d be nice to have is the ability to flip pages using the arrow keys on the keyboard. Rn you still have to use your finger or, if on an external display, a mouse/cursor.