unum illud certissimum (sc. genus epistularum)
res ipsa: i.e. the practice of letter writing
aliquem certiorem facere: inform someone
absentis: accusative plural (the regular ending in C)
quod: refers back to quid and is the boject of eos scire
quod eos scire: clause is the subject of interesse
nostrā: ablative with interesse
ipsorum: genitive with interesse
interesse: see Lewis and Short on this verb (with ablative or genitive of person)
huius generis: of this sort
domesticos: with both scriptores and nuntios
novi: partitive genitive with nihil
magno opere: also written magnopere: adv: greatly
utro…uti: from utor,which takes an ablative of means
deceat: indirect question
iocer-ne: deliberative subjunctive
temporibus his: i.e. during the current political upheaval
