I have to correct my previous post.
I found out why the book did not finish encoding. There was an unrecognized HTML code in my document, and for some reason Bookdesigner canceled encoding the rest of the book.
I've corrected the issue, and without problems encoded a 11,6MB lrf file without error.
you can find it
here
Perhaps I was too soon to boast hitting the limit..
/Me goes off to the library to find an even larger book!