FM11 book file opening error with FM10

I don't have Fm11 on my machine anymore, but you can generally use Save As to save back to the previous version. 

So, while the MIF solution will work, it may be easier/faster to Save As Fm 10.

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.

No.

But you can save the FM11 files to MIF and open the MIF file in FM10.