If you want to move all of your book as well as the Collections data you had for them from one computer to another you’ll want to copy all of these files from the old computer to the new one:

• ~/Library/Containers/com.apple.BKAgentService
• ~/Library/Containers/com.apple.iBooksX

It’s possible you only need to move the specific Books folder here:

Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books

But it seemed to work okay to move the entire contents of the BKAgentService folder, but there are definitely non-iBooks related content in there.