FM 10 crashes when you try to use CTRL+F4 to close a book

In FrameMaker 10, if you try to close a book using the standard key sequence CTRL+F4, FrameMaker will crash with internal error 10004 7685768 7686058 10067517.

CTRL+F4 worked in FrameMaker 9 and it works in 10 when trying to close open documents.

I don't understand why FM 9 and 10 do not have a standard close button [x] on the book tabs as was the case in earlier versions. It makes it cumbersome to close books if you usually work with several open.