Why is iBooks.app taking 650 MB memory on macOS 10.14.5?
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.