Epictetus, of course, wrote nothing. The primary work for his philosophy is the collection of the Discourses of Epictetus collected by Arrian, one of his students. The exact relation of the Encheiridion to the Discourses is unclear. It would be plausible to think of it as an abstract of the ideas of the Discourses, but not all of the content of the Encheiridion finds any clear correlation with ideas in the Discourses. This may be because we are missing some of the books of the Discourses (we have four books, and there are some indications that there were originally eight). Or it may be that the relation between the two is a bit more complicated than has usually been assumed.
You can read the Encheiridion online in English in the eighteenthicentury translation by Elizabeth Carter or in the nineteenth-century translation by George Long at the Perseus Project. Perhaps the most important commentary on the Encheiridion is that of Simplicius, which can also be found online.
(1) Some things are in your power and some things are not. The very first sentence of the Handbook gives us one of its major themes. Some things are up to us -- our opinions, judgments, desires, aversions -- and some things are not up to us -- our bodies, our possessions, our reputations. Being able to distinguish between the two is a considerable part of freedom. Misfortune is not getting what you desire, so the person who is free of desire for things not up to him is someone who can never be unfortunate, "but if you are averse to illness or death or poverty, you will meet misfortune" (#2), because these things are not up to you. Because of this, whether your life is a good one depends in great measure on accepting that things outside your power really are outside your power: "Do not seek to have events happen as you want them to, but instead want them to happen as they do happen, and your life will go well" (#8). If you go lame, it is your limb, which is not up to you, that has been interfered with; your will, which is up to you, is still intact (#9).
Likewise with everything around you: "You are foolish if you want your children and your wife and your friends to live forever, since you are wanting things to be up to you that are not up to you, and things to be yours that are not yours" (#14). We should act as if we were at a banquet (#14), not constantly reaching out our hands toward everything, but simply letting it come to us, if it does. Likewise with your role in society. We should be like actors in plays (#17), playing whatever role is assigned, and doing it according to our own skill, because we are not the ones who write the parts. (It is always worth remembering that Epictetus began his life as a slave.) And it is only by giving up on trying to control things we cannot control that we free ourselves of envy and live unconquered lives (#19).
As far as the things we cannot control go, everything happens as it should: targets are not set up to be missed (#27) and piety toward the gods requires acquiescing in what happens (#28). Impiety follows, in fact, from taking our good or bad to lie in things that are not up to us. Likewise, if people treat you badly, you need to remember that they cannot see things as they appear to you, but only as things appear to themselves (#42), and it is not you who are actually harmed by it. You should simply not worry about what others say about you, because this is not up to you (#50). You would recognize the absurdity of someone turning over your body to be tormented by anyone who came along, so why would you go about turning over your own mind to be tormented by anyone who came along, just because he insults you (#28)? No one can do you harm unless you turn your mind over them to be harmed (#30).
(2) Appearance is not reality. Our connection to the world is mediated by phantasia, appearance or impression, the sense and feel of it, so to speak. But appearance is not reality. Thus, when faced with an appearance, we should recognize that it is not the thing that appears. Instead of going how things appear, we should use reason to take into account the way they really are. Much of human misery comes from being carried away by appearance, in a way not appropriate to, or even inconsistent with, reality. Whenever we are faced with some sign usually taken to be of something bad (Epictetus uses omens as his example, but obviously applies to any kind of sign), we should recognize that the sign is a matter of appearances, for the use of things external to us, like our body or our property, or other people (#18). Whether it is really of something bad depends entirely on us, because, whatever it may signify, we may make the choices that allow us to benefit from it, no matter what it is -- and so it is in our power to make all omens good omens.
If something makes you afraid or miserable, it is your own opinion that makes it fearful or terrible; you have only yourself to blame if you are afraid or miserable. If you put your joy in a matter that is not up to you, like the beauty of your horse (#6), you are treating something over which you have no power as having power over you, and so you are enslaved. When someone is grieved by the loss of property or the death of a child, what actually weighs them down is their judgment about what has happened, which is based on the appearances (#16). If another person irritates or insults you, avoid being carried away by appearances by delaying your response until you can control yourself (#20). And we should be careful of insinuating our evaluations into things. If someone drinks a massive amount of wine, we should simply say that he drinks a lot of wine, not that he does badly; whether or not he does badly can only be determined on the basis of his judgment or actual decision, not on the basis of the appearances (#45).
We learn to recognize the will of nature, as opposed to appearance, by universalizing (#26). If someone else accidentally breaks a cup, we can all recognize that this is just a thing that happens, and one must bear it; so we should learn that this is true if we break our own cup. If a stranger's child dies, we recognize the awfulness of the appearance, but we are not carried away by it; we recognize that it is the way of things. And so, too, we should remember this when we face our own losses. But the same is true for pleasant appearances; they too must not be allowed to carry us away. Here we should also wait for an appropriate time, and delay our actions until we can control ourselves (#34).
(3) The philosophical life is a life of internals. All of this directly establishes that the philosophical life focuses on the internal rather than the external. We should not be concerned about what other people think about our choosing to live philosophically; we should rather simply do it so that we may have freedom and self-respect. When we consider something that we'd like, we should think it through. Suppose, for example, that you want to be victorious at the Olympic games (#29). You should not simply consider the victory but what leads up to it -- the discipline, the difficulty, the work -- and what may follow from it -- injury, losing -- and make your decision in light of that. You cannot be a philosopher by imitating philosophers as if you were a monkey; you must understand what is involved in it and act accordingly. Too many people hear or read some philosopher and think that if they imitate the way a philosopher talks that they have become a philosopher. Far from it! This is like thinking you can be victorious in the pentathlon without any training. To be a philosopher, you must be willing to trade everything you might lose -- honor, public office, reputation, wealth, or whatever -- for the freedom and tranquility of rational life. If you focus on the externals of philosophy, this is precisely what it is not to be a philosopher. The philosopher looks for benefit or harm to come from himself, not from anything outside (#48). If you want to be a philosopher, practice philosophy, not something else.
We should indeed take Socrates as our model, not in externals, but in his attitude toward things. If we want to know how to deal with a situation, we should ask what Socrates would do (#33). Socrates shows us that death is not dreadful in itself, by the way he died (#5). If we are looking for signs, we should approach them as Socrates would, giving priority to reason and only using them as supplements (#32). If you visit someone important, you should already be reconciled to the possibility of not meeting them (#33). Like Socrates, we should not make any ostentatious display of how philosophical we are; he was so far from ostentation that when other people asked him to introduce them to philosophers, he did not put himself forward, but took them to other people (#46). With anything that comes up, we should recognize that in our lives we are like athletes in a great contest and that every moment in the game matters. Socrates did this and became perfect in his own way; if we are not like Socrates, we should nonetheless live like people who want to be like Socrates (#51).
The Handbook ends with four quotations that Epictetus recommends that we keep in mind at all times. The first is from the Stoic philosopher Cleanthes:
Conduct me, Zeus, and thou, O Destiny,
Wherever your decrees have fixed my lot.
I follow cheerfully; and, did I not,
Wicked and wretched, I must follow still.
The second is from Euripides, that very philosophical writer of tragedies:
Whoe'er yields properly to Fate is deemed
Wise among men, and knows the laws of Heaven.
And the third and fourth are from Plato's Socrates (the Crito and the Apology, respectively):
O Crito, if it thus pleases the gods, thus let it be.
Anytus and Melitus may kill me indeed; but hurt me they cannot.
Quotations are from Epictetus, The Handbook, Nicholas P. White, tr., Hackett (Indianapolis: 1983), except for the quotations at the end, which are from the Higginson translation at Perseus Project.