How do I change the apple ID linked to iBooks in a mac?

I recently purchased a new Macbook Air and I gave my old one to my brother. While setting the old mac for him, I wasn't able to unlink iBooks from my Apple ID, and link it to my brother's. This was fairly easy to do with all other apps (Facetime, iMessage, iCloud, etc), but not with iBooks. I tried to do this in "Preferences", but there was no field or command to change user for iBooks. I also looked inmy account's settings using the iBooks store link, but the fields allowed me to edit my own Apple ID instead of logging out and log back in with my brother's ID.

Any ideas on how to solve this problem?