At least this is the case for me under the Mojave OS. I have defined four spaces in Mojave on my MBP.
Once in awhile I think the Books app is behaving itself and showing me any open Books window only in the space I've specified in the Dock options for that app.
But sooner or later when I'm switching spaces and expecting to see only say my browser, or an open PDF file in some other space, then kaboom, the Books window muscles in and lands in foreground (always ends up in front) so I have to either hide the app or manually bring a desired app's window(s) forward.
Then if I switch back to where I expect Books to be, of course it's not there if I've hidden the app, so I have to swoop down to click on the Books app in the dock to bring it back up into the space. And yeah, if I then switch through my spaces, the Books app is once again front and center in every single one of those spaces.
So far I haven't managed to construct a replicable scenario for the initial fail after a launch of Books, i.e. take note if the fail occurs only after I've selected some space that has a certain other app running in it, or only if I just have a particular book open, not the Books library window, etc. I get too annoyed to try to do that but maybe it's time. I figured to start a more formal investigation by wondering out loud in this forum if it's an experience others have found in using the Books app on a desktop or laptop running Mojave or in Catalina for that matter.
Once in awhile I think the Books app is behaving itself and showing me any open Books window only in the space I've specified in the Dock options for that app.
But sooner or later when I'm switching spaces and expecting to see only say my browser, or an open PDF file in some other space, then kaboom, the Books window muscles in and lands in foreground (always ends up in front) so I have to either hide the app or manually bring a desired app's window(s) forward.
Then if I switch back to where I expect Books to be, of course it's not there if I've hidden the app, so I have to swoop down to click on the Books app in the dock to bring it back up into the space. And yeah, if I then switch through my spaces, the Books app is once again front and center in every single one of those spaces.
So far I haven't managed to construct a replicable scenario for the initial fail after a launch of Books, i.e. take note if the fail occurs only after I've selected some space that has a certain other app running in it, or only if I just have a particular book open, not the Books library window, etc. I get too annoyed to try to do that but maybe it's time. I figured to start a more formal investigation by wondering out loud in this forum if it's an experience others have found in using the Books app on a desktop or laptop running Mojave or in Catalina for that matter.