One example of such a modal particle is mal in
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.)