The author is unknown. It was written in 64 AD. It is an episle. The theme is Jesus is our High Priest and God. The book of Hebrews was written to Jewish Christians. They first knew God through the old covenant, but now are born again and are introduced to God through the new covenant. This caused confusion.
The book of Hebrews establishes and outlines Christianity being the natural succesor to Judaism and Jesus being superior to prophets and angels, and the past revelations of God in the Old Testament. Jesus is the fulfillment of the Law. Hebrews is among the most systematic and doctrical books within the New Testament, along with Romans.
Hebrews engages the very real danger to these new Christians. As Christianity was increasingly regarded as a cult by the Jewish groups, as they started to experience brutal persecution, there was a strong temptiation for the Jewish Christians to return to Judaism and forsake Jesus and Christianity. There were also attempts to make Christianity more Jewish, therego making it more acceptable to the Jewish establsimment. This book addresses and corrects these errors. Christianity flowed forth by the natural growth from Judaism, yet it was greater. It is the new covenant.
Most likely, this book was written prior to 70 AD, when the temple and Jerusalem was destroyed. A book which has much to say about the temple, such an event most likely would have been mentioned. Church tradition atributes the author being Paul the apostle, and this is who we beleive is the author. Its style is unique somewhat to Paul, although it is to be expected since his audiance was unique compared to the rest of his epistles. Hebrews obviously contains a lot of teaching which agrees with the rest of Paul’s letters, and based on Hebrews 13:23, the author served with Timothy. Regardless whether or not Paul was the human author, what is most important is that the Holy Spirit inspired this book, and it is a blessing full of a treasure trove for us, and it illustrates a graphic picture of Jesus.