http://bible.org/article/non-pauline-epistles
Writing about the book of Hebrews:
Many suggestions have been made for the author of this anonymous book—Paul, Barnabas, Apollos, Silas, Aquila and Priscilla, and Clement of Rome. There are both resemblances and dissimilarities to the theology and style of Paul, but Paul frequently appeals to his own apostolic authority in his letters, while this writer appeals to others who were eyewitnesses of Jesus’ ministry (2:3). It is safest to say, as did the theologian Origen in the third century, that only God knows who wrote Hebrews.75
I think it’s fascinating to have this book which is accepted as canon, The Word of God, and we don’t know who wrote it. That may be true of OT books as well, for that matter. The theology and text of the book are sufficient to have it be accepted into the circle of inspired works.