So here is the list of Seneca's letters I have have drawn up based upon suggestions and with a couple of additions. Please let me know in comments if you have any changes or additions or..well, comments!
1. On Saving Time
4. On the Terrors of Death
16. On Philosophy, the Guide of Life
19. On Worldliness and Retirement (or why go back to Plato's cave once you have left it?!)
41. On the God within us
49. On the Shortness of Life (cf. Bede's Sparrow)
58. On Being
87. Some Arguments in Favour of the Simple Life
97. On the Degeneracy of the Age
119. On Nature as our best Provider
123. On the Conflict between Pleasure and Virtue
Let me know what you think. I am off to delve into Aristotle's Physics Book VI..which looks very crunchy indeed.
There is also a book which I will be looking into called: 'Senca as a philosopher in politics' Miriam Griffin OUP 1992, which tries to construct Seneca's career and place him as a philosopher within the context of Roman statesmanship of the transitional period between the Republic and the Principate. Some boning up on Neronian statecraft and realpolitik might also be useful but..as Seneca reminds us..time 'aint long.