This is a great sermon during this Advent season about angels called "The Ministry of Angels". The pastor began by saying how many times angels are sung about in our traditional Christmas Carols. If you stop to think about it, they really are mentioned a lot.
Angelology is a fascinating study, and there is much in the Bible about angels. But like most doctrines, it can and often is twisted by the unstable. It is a target for fringe theologians who spout fanciful theories and supposed visions. As Phil Johnson has said, "Few biblical topics have provoked more wild speculation and fruitless debate than the topic of angels."
Because of this I am providing what I have found to be safe and solid material on angels for further study. After listening to the sermon I linked above, there are other great resources that explore the facts about these incredible beings.
John MacArthur has several pieces about the different ministries of angels, including angel armies, (Angels: God's Invisible Army, Part 1 with other parts after this), entertaining angels (Ever Met an Angel?), Angelic Messengers, and others. Search for angels at gty.org.
Phil Johnson wrote an essay at Ligonier called Angels: Messengers and Ministers of God.
Martyn Lloyd-Jones preached two sermons on angels in his Great Doctrines series. One is called Good Angels. Sadly, there is no recording of his subsequent lecture titled "The Devil and the Fallen Angels" but there is a transcript you can read which I uploaded for the public. Here is the link to the transcribed sermon The Devil and the Fallen Angels by Lloyd-Jones.
Randy Alcorn, author of the book 'Heaven', has several articles about angels at his website. How Do We Know That Seraphim and Cherubim Are Angels?
Alcorn also answered this question In Their Own Realm, Do Angels Have Physicality?
Spurgeon’s Angelology: A Speculative Thought.
In this sermon, Charles Spurgeon discusses the angels who ministered to Jesus after His temptation in the wilderness. Satan departing, angels ministering
RC Sproul preached a two-part series simply called Angels.
Now here's a little debate for you: do angels sing? I wrote about this a few years ago. Despite the hymns and carols noting that angels sing this or that, there is very little scriptural evidence that they do sing. But no scriptural evidence claiming they unequivocally do not sing. Hmmm.
GotQuestions: Do Angels Sing?
The End Time: Do Angels Sing? In which I look at the stances of some theologians who believe they do or do not, as well as offer some food for thought.
In the end, lest we fall guilty of what Phil Johnson said above, that we engage in wild speculation or fruitless debate, let me end here with a link to a page listing scriptures in which angels are mentioned, and it says there are 499 scriptures (but 25 are noted here).