Fit-to-Screen Issue in Aperture Books / Albums
I changed my split-view for building albums, put it back and then lost my "100%" button and the button that lets you fit the album to the size of the screen. How do I fix it?
Are you asking about these buttons?
This might have been caused by a corrupted preferences file.
Then quit Aperture and move the file ~/Library/Preferences/com.apple.Aperture.plist from your User library in your Home folder to the Desktop.
See this document Aperture 3: Troubleshooting Basics on how to open the User Library and to remove the preferences file.