Rick Quatro sells an ExtendScript that will mass convert files and books to earlier FM versions or to MIF, or to more recent versions of FM. If you need to convert files between versions on a regular basis, or just need to convert a whole LOT of files one time, it might be worth the $49 for a license.
Lin is correct. You will have to save the book file and all its components as MIF, one at a time, in FM11 (or later). FM10 (or earlier) can then open the MIF files.