How to romanize the title of a book entitled Sefer Mahaneh [dalet - nun sofit] by David ben  Avraham Nahmias?

This matter is addressed in HCM, p. 31:  "Other [that is, other than acronyms in common use and rabbinic names like Rashi] Hebrew abbreviations that are in common usage as words are ... romanized as such."  This fits the above case:  "Dan" (or "dan") is certainly "in common usage as a word".  We simply, then, "romanize them as such," and take no notice of the
geresh in romanization (though of course we include it in vernacular transcriptions).