These are standard abbreviations in classical scholarship. N.Q. is Seneca's Naturales quaestiones, Ep./Epp. are the very Epistulae Morales ad Lucilium you are reading (respectively, singular/plural), Vergil, Georg. is Vergil's Georgics, Cicero, De Nat. Deor. is Cicero's De Natura Deorum, etc. You can typically find the full title by googling the abbreviation, in scholarly books they would typically be listed at the beginning or the end. The Roman numeral is the letter or section number, the Arabic numeral is the line or paragraph number, in many editions they are explicitly labeled in the text; f./ff. (from Latin folio) mean the following page/s or paragraph/s.
Some general lists of abbreviations available online are Yale Classics, Oxford Classical, and Oxford Latin. Unfortunately, none is comprehensive. Some books, for example, Herod and Augustus, pp. ix-xiv have a list of abbreviations specific to them. See also Abbreviations for Classical authors and texts for general information.