In Goodman and Gilman's The Pharmacological Basis of Therapeutics 12th Edition pg. 406, two drugs, nor1-citalopram and nor2-citalopram are referred to but I have no idea what they are. Could somebody tell me a more commonly used name of these drugs or their IUPAC names if common names do not exist. Also please show me what exactly the nor1 and nor2 refers to. I'm guessing there's methyl groups missing in them at different positions because I know that norpethidine (or normeperidine) refers to the N-demethylated metabolite of pethidine (or meperidine).
Those names are referring to desmethylcitalopram and didesmethylcitalopram, which, as you correctly predicted, are the N-demethylated and N-didemethylated metabolites of citalopram. Per IUPAC conventions, the nor- prefix is used to refer to any structural derivative of some parent compound in which a single carbon and (possibly) its hydrogens have been replaced (typically by a hydrogen, but not exclusively). See the relevant Wikipedia page on the prefix for more detail.