Different opacities for graphics in the book layout

@Mateomono 

As others said - you need to speak with people that will be printing it - so they won't try to make your graphics darker.

Using grayscale to add clarity and "range" to nominally monochrome graphics is a very standard method. (I mean... photos? 🙂 )

But as BobL says, you should check with your (actual, production) printer for recommendations on minimum density, recommended steps of density to differentiate levels of gray, and linescreen. This is not generic or guess information; it needs to come from your specific printer/print service.