Ball-Inventor can be built with just one set of 51515 Mindstorms Robot Inventor. No additional parts needed. 
The marble run can be used with just the 16.5mm red ball included in the set. Of course it is more fun to run it with several (different) marbles. LEGO Soccer- or basketballs can also be used. Even better are normal glass or metal marbles with diameters up to 16.5mm.
The actual downward slope can be changed according to your preference. If you own some more technic panels, you can further extend it to create more interesting runs and let the marble go different ways each time. You can use the “Mixer” motor to force the marble into a different  (random) direction each time.
The Building instructions on rebrickable.com include detailed explanation of the WordBlock program as well as some theory on motor positioning. The program is provided as an .lms file which you can import into the Mindstorms app.
All instructions are provided in English and German.

More Details on the page about the book The Mindstorms Robot Inventor Expert