Of the early Christian writings that didn't make it into the New Testament, the one most worth reading is probably 2 Clement. Probably written in the early second century, it's really the oldest surviving non-biblical sermon. It's certainly worth a read. Particularly interesting are the quotes from Jesus that are not found in our gospels... 2 Clement was written at a time before the four gospels as we have them were accepted as standard. Obviously Jesus said heaps of things in his life, not all are recorded in our gospels so it is fully possible that the words the author quotes are authentic words of Jesus.
Here are his quotes from Jesus. You might well recognise some as very similar to things found in the gospels:
Jesus: “Even if you were nestled close to my breast but did not do what I have commanded, I would cast you away and say to you, “Leave me! I do not know where you are from, you who do what is lawless.””
Jesus: “You will be like sheep in the midst of wolves.”
Peter: “What if the wolves rip apart the sheep?”
Jesus: “After they are dead, the sheep fear the wolves no longer. So too you: do not fear those who kill you and then can do nothing more to you, but fear the one who, after you die, has the power to cast your body and soul into the hell of fire.”
Speaker unclear, but it could well be Jesus: “How miserable are those of two minds, who doubt in their hearts, who say “We heard these things long ago, in the time of our parents, but though we have waited day after day, we have seen none of them.” Fools! Compare yourselves to a tree. Take a vine: first it sheds its leaves, then a bud appears, and after these things an unripe grape, and then an entire bunch fully grown. So too my people is now disorderly and afflicted; but then it will receive what is good.”
Jesus: The kingdom will come “When the two are one, and the outside like the inside, and the male with the female is neither male nor female.”
According to the author of 2 Clement, this saying means that the kingdom will come when there is no hypocrisy, when the goodness of our souls equals the goodness of our deeds, and when we make no distinction between sexes [perhaps meaning we put aside lust?].
Jesus: “Woe to the one who causes my name to be blasphemed.”