These are early church documents from the Apostolic Fathers. Since they are not scripture, naturally the doctrine suffers. The only of these documents that I would recommend as sound doctrine is the 1st Epistle of Clement to the Corinthians [ca 96]. The letter is not without a few glaring inaccuracies, however. As for the rest of the letters, they have good teaching in places, but overall are lacking in accuracy. I will need to read the letters in more detail to make any more recommendations. The letters of Ignatius are difficult to judge with regard to doctrine, because they do not have much in the way of teaching.
I've tried to limit these documents only to those written before 200 A.D. They came originally from a more complete list of Early Church Documents. However, some bits of editing were needed to format some of the links for the Web and correct errors in italicizing. I am referring to some of the letters referenced in the links, not portions of the NJB early church guide, which follow below.
1st Epistle of Clement to the Corinthians [ca 96]: A formal letter written on behalf of the Roman Christian community urging Christians who had been rebelling against church authority to be submissive and obedient. Tradition attributes it to Clement, allegedly one of the first bishops of Rome.
2nd Epistle of Clement to the Corinthians [ca 150]: Sermon thought not to be the writing of Clement himself. Advocates sound view of Christ, the resurrection, and holiness unto God. Enter into battle against the ways of this world, work out salvation through strength in Christ.
The Epistle of Barnabas [ca 130]: This letter, probably not authored by the NT Barnabas, repudiates the claims of Jewish Christians at the time who advocated adhering to observance of the Mosiac Law. Argued that Christ provided salvation and man is no longer bound by the Law. Compares holy life to unrighteousness. The last part of this treatise consists of a form of the Two Ways Teaching also found at the start of the Didache.
The Shepherd of Hermas [ca. 150]: Written by Hermas, who is believed to be brother of Pius, the Bishop of Rome. The Shepherd of Hermas is an apocalyptic document (in the sense that it claims to be revealed), modelled after the Book of Revelation. It deals with practical matters of church purity and discipline in second century Rome.
The Epistle of Polycarp to the Philippians [ca 130?]: Polycarp was a church leader (bishop) in Smyrna, Asia Minor. Exhorted the Philippians to holy living, good works, steadfast faith. Interested in ministry and practical aspects of daily life of Christians.
The Writings of Ignatius: Bishop of Antioch in Syria [ca 1-2 century] martyred in Rome by beasts (ca 105-116). On his way to Rome, he visits and then writes to various churches, warning and exhorting them. He also writes ahead to Rome, and writes to Polycarp, bishop of Smyrna. Warned the church against heresies that threatened peace and unity, opposed Gnosticism and Docetism. In the Epistle to Smyrna, insisted Christ came in the flesh not just in spirit.