I have made up a one-page handout cross-referencing Senecas Epistulae Morales against the various 20th-century commentaries, each of which covers a different selection. The Word 2000 for Windows (.doc) version is here, the Adobe Acrobat (.pdf) version here.
Besides showing at a glance which commentaries cover which letters, and (very interesting) which letters are more or less popular among commentators, I also use it as a check-off list to keep track of which ones Ive read. With 124 letters and a substantial fragment of another, its easy to lose track. If anyone wants to see how I did it, or is curious about which ones Ive read, the color-coded personalized versions are here (.doc) and here (.pdf).
Feel free to print out any of these files and use them yourselves. As always, comments and queries are welcome. I should probably mention that the unfortunate language in 47 and 56 is a purely coincidental result of listing commentators by the first letters of their last names and in order of publication.