In "Moral Essays volume 2".
Online version: http://www.stoics.com/seneca_essays_book_2.html
On the same site (http://www.stoics.com/books.html) there are versions of:
Discourses
Online version: http://classics.mit.edu/Epictetus/discourses.html
Discourses and Selected Writings (Penguin Classics), new translation by Robert Dobbin, includes Enchiridion and "fragments" (amazon.com).
Enchiridion (Handbook or Manual)
Online version: http://classics.mit.edu/Epictetus/epicench.html
(The so-called “Golden Sayings” is a later compendium of aphorisms drawn from the Discourses and Encheiridion.)
Complete work online: http://www.davemckay.co.uk/philosophy/epictetus/
Meditations
Online version: http://classics.mit.edu/Antoninus/meditations.html
The Stoics 2nd ed. F. H. Sandbach Published 1898 by Bristol Press in London . http://openlibrary.org/books/OL24151317M/The_Stoics
The stoic philosophy By Murray, Gilbert http://openlibrary.org/works/OL1243262W/The_stoic_philosophy http://www.archive.org/download/stoiephilosophy00murrgoog/stoiephilosophy00murrgoog.epub