One example of such a modal particle is *mal* in

> 
> Schau mal.
> 
> 
> 

Another example is *denn* in

> 
> Was soll denn das?
> 
> 
> 

Spoken German is awash in such modal particles. My question about them is always the same: *how exactly does the meaning of the utterance change when that modal particle is omitted?*

Does anyone know of a book ***that addresses this question?***

(I'd prefer a book in English, but one in German would be OK too. Spanish and French would also be OK.)