Accessing backed-up material?

I have a G-drive SSD USB-C 500GB for backing-up a MacBook running Sierra.
My problem is that when I try to access the G-Drive in order to retrieve a book which has disappeared from current iBooks I get a message stating that iBooks cannot be accessed because it is ‘in use’. What does this mean, it is on a separate hard disc, & how can I get the book back from the back-up? Please keep advice simple, I am not very computer literate.Thanks.

In order to access the data you’ve backed up you need to go through the Time Machine restoration process. You cannot just open the drive on the desktop and access files. For more in depth assistance with Time Machine you’ll need to contact Apple since it is their software built into the Mac.

Lydia,
Thank you.
I do feel that WD should provide a bit more info with the device itself instead of expecting buyers to have to go to a web site to find how to use it. Best wishes, Dennis.