Was Martin Luther known to have read the First Epistle of Clement? The First Epistle of Clement is an early Christian document that was commonly circulated in the early church before being rejected as part of the New Testament canon. There is a passage in it that addresses faith and works that sounds rather reminiscent of Martin Luther's theology 1500 years later (emphasis mine):


  All these, therefore, were highly honoured, and made great, not for their own sake, or for their own works, or for the righteousness which they wrought, but through the operation of His will. And we, too, being called by His will in Christ Jesus, are not justified by ourselves, nor by our own wisdom, or understanding, or godliness, or works which we have wrought in holiness of heart; but by that faith through which, from the beginning, Almighty God has justified all men; to whom be glory for ever and ever. Amen.


It then goes on to talk about the value of works, but doesn't say that there is justification from it.

Is Martin Luther known to have read the First Epistle of Clement? And if so, did he comment on what he thought about it?
No. Luther is not known to have read the First Epistle of Clement. In fact it is certain that he did not read it.

Luther was, in general, very well read in the Church Fathers; and he would have known, from Eusebius and others,  that such an epistle had once existed. However it  had long since disappeared, and no copies were known to exist. It was only in 1628, more than 70 years after Luther's death, that Western scholars were able to read it. This happened when  Patriarch Cyril I, of Constantinople, presented King Charles I of England etc. with a copy of the Bible containing, separately at the back, a copy of Clement's Epistle. This became known as the Codex Alexandrinus, because it had been found, a few years earlier, in Alexandria.  The Patriarch had intended it to go to King James,  but he died before it arrived. It was published by Patrick Young, the Royal Librarian.  

Doctor Bell in his translation of the Familiar Discourses of Luther lists some, though not all, of the Early (and later) Fathers Luther was familiar with including a date when they were thought to have been active.

Athanasius of Alexandria (387) 
Basilius Magnus and Gregorius (380)
Ambrose of Milan (380)
Aurelius Prudentius (380)
Austin of Hippo (430)
Beda, a Benedict in England (737)
Cyprian of Carthage (259)
Cyril of Alexandria
Chrysostom of Constantinople (420)
Gregory I, Pope (511)
Hierem, eldest of Striden (522)
Irenaeus of Lyon (175)
Polycarpus, tutor  to Irenaeus (1&amp;5)
Nicolas de Lyra (330)
Origen of Adomantz (261)
Philo the Jew (50)
Josephus (100)
Ignatius of Antioch (111)
Prosperus of Rogen (460)
Sedulius (430)
Tatianus the Heretic (170)
Thomas Aquinas  (1274)
Bonifacius of Mentz (755)
Bernard of Cistern (1140)
Hugo Parisiensis (1130)
Anselmus (1110).


Further detail is included in Staniforth Maxwell's Early Christian Writings and the Apostolic Fathers. This link is to a preview. The pages are not numbered but it is on the twelfth page.