Why is iBooks.app taking 650 MB memory on macOS 10.14.5? I am using macOS Mojave 10.14.5. Books.app shows 650 MB memory being used in Activity monitor even when it is not opened. I have books of around 110 MB but I haven't used the app very much recently. 


I tried opening and quitting the app. And quit the process from activity monitor too. It didn't stop. Then, I had force quit the process from Activity Monitor. It fixed it for some days and now it is back in the same state. 
In the "Open Files and Ports" of Activity Monitor, ~/Library/Containers/com.apple.iBooksX/Data/Documents/BKLibrary/BKLibrary-1-091020131601.sqlite is an entry along with .sqlite-wal and .sqlite-shm with the same file name.
I booted into Safe mode and worked for about 30 minutes during which the app didn't appear in Activity Monitor. It came back in normal mode after around 2 days. 
New find is that, when I open iBooks, another PID of the same app is created with around 50 MB. All the further actions like, opening a book, closing it, networking processes etc affect only the newly created PID. Quitting the app kills the newer PID, not the older one.


What is triggering books.app to start and use that much memory?
I note that Books is using a similarly small amount of memory on my Mac, too, despite not being launched. So, I'd categorise this as 'normal'. 

My general approach to memory issues is "Is this an actual problem?" 

If memory pressure is low; and you're not seeing high levels of swap and associated  performance issues: then I'd be tempted to let MacOS do the system management. 

You could try launching the app, and letting it 'work' for a bit, then quit it, and see if that improves things. There do seem to be a couple of other reports on ASC about Books running wild with memory - the DiskSpaceEfficiency plugin seems to be the culprit. But no solutions given. FWIW, I don't see the behaviour in Catalina. It's likely to be a bug.