Here is a simple way for multiple libraries
Say you have two libraries, your current, and another one you intend to create. Let's say the second library is "Library 2". Make sure it is empty.
Open Calibre, and go to Preferences--> General
Now change the library location to "Library 2". Click OK.
Calibre now will copy all your books from the current library to Library 2. In Calibre, delete the books you don't intend to keep in Library 2. (Don't delete them in the folder --- you need to use Calibre to delete them!)
Now you can add books as you wish in Calibre. Whatever you have added will be in Library 2. To go back to your original library, go to Preferences and change the library to your original library. Note that you may have to quit Calibre first and restart it. Somehow on my computer Calibre only allows me to switch library once without quitting it.