Author: Joel. There are a dozen different men with the name of Joel in the Old Testament but the prophet Joel appears to be distinct from all of them. Little is known of him aside from his father is named Pethuel whom we also know nothing about. Outside of the book of Joel itself, the only reference to Joel (the man, not quotations from the book) is in Acts 2.
Date: It is hard to give an exact date with the book of Joel as there are no markers within the book to compare it to. However, many biblical scholars place the book in the ninth century BC. This would make Joel a probably contemporary with Jonah and Amos. Regardless of which of these prophets actually spoke first, Joel is the first prophet to speak to the southern kingdom of Judah. On our timeline of the prophets, Joel is estimated to minister around 800 BC but this should be considered only a rough estimate.
Recipients: Joel clearly has the southern kingdom in mind as he focuses on Judah and the city of Jerusalem throughout the book.
Key themes: Joel 1 records an invasion of locusts while Joel 2 speaks of an army of locusts. Some see these locusts as metaphors for the coming Babylonians, Medo-Persians, etc. However, given the context of the rest of the book, a literal plague of locusts is plausible and is the more conservative interpretation.
Joel 2:28-32 prophecies the coming of the Holy Spirit upon all believers. This section is quoted in Acts 2 as being fulfilled on the day of Pentecost.
Joel 3:17-21 looks to a time of future blessing of Israel. This is likely a depiction of the Millennial Kingdom as it has not been literally fulfilled to date.
Key phrases: The “Day of the Lord” appears several times throughout the book. This is both a day of judgment for unbelievers but also a day of joy and blessing for believers.