I thought _Sourcery_, but I'm not seeing anything 
at <
http://www.lspace.org/books/apf/index.html>
I rarely drink coffee or alcohol.  Is consuming the 
two together unremarkable - from one cup or from two?  
I've heard of "Irish coffee" as another case, but 
"caffe corretto" seemed to be meant as special...
oh, hang on: 
<
http://en.wikipedia.org/wiki/Caff%C3%A8_corretto>
"In Spain, a similar drink is known as carajillo, 
and in Sweden, Norway and Denmark as kaffekask, 
karsk and kaffepunch, (although "karsk" or kaffe 
is in fact coffee added to moonshine to make the 
moonshine palatable, rather than liquor added to 
coffee for flavor) respectively."
Maybe everyone /is/ at it?  Or is the one drink 
to neutralise the other a possible specific 
reference?  
It's early in the canon, so it might be not so 
carefully done.
An Islamic country (which a Discworld country 
usually isn't) is unlikely to use alcohol 
except medicinally, but as a remedy for the
coffee, it could technically qualify as medicine.
Having said that, "Arabian Tea" is quite something.
<
http://en.wikipedia.org/wiki/Khat>