🚀 go-pugleaf

This is how you do it:

Outside of FM, copy the "Book A" folder and paste it where you want.
Also, create the Graphics folder and file structure (i.e.
"... Graphics Catalog B\Graphics ...")
Before you open Book B in FM, temporarily rename the graphics
folder that the FM files in Book A uses, e.g. "Graphics Catalog Z".

Now, when you open the files in Book B, FM won't be able to find
the old graphics since the folder name has changed. For each file,
FM will open the "Missing graphics" dialog, where you need to
browse to the new graphics folder, e.g. "Graphics Catalog B\Graphics".
Select the correct file that FM asks for, and tick the option to
use this folder for all remaining graphics.

If all the image file names are identical, you will only need to
do this once per FM file. If you really need the image file names
to differ like you said:
> ..\..\..\Graphics Catalog A\Graphics\a000123.png
> ..\..\..\Graphics Catalog B\Graphics\b000123.png
then you need to select the correct file name for *each* graphics
that FM cannot find. This is a good reason to keep the file names
identical, even though the folder name has changed.

When all the files have opened, rename the original graphics folder
back to "Graphics Catalog A".

-- 
/Thomas Michanek