Mea culpa, this is really embarrassing: accidentally I placed an Alias into the plugin folder instead of the file itself. Now with the script in place it works well and I can import the obj into modo.

I also realized that the word 'book' doesn't occur anywhere in the script, but having no idea about SU programming I was confused.

Thanks a lot for your quick help and this great script!

I use a Mac OSX 10.7.3 with SU 8.0.11751. OBJexporter.rb is in the 'Library/Application Support/SketchUp/Plugins' folder. When I start SU 8 I get a pop-up window "Load Errors" which says:

Error Loading File OBJexporter.rb
undefined local variable or method `book' for main:Object

Did I miss something, I am really new to SU? I see people here having OS X and SU8 without problems, so it should work.