My Angel Will Go Before You by Georges Huber with an introduction by Cardinal Journet
Angels have a hard time in modern culture. Either they are considered pals who get kids out of trouble or their existence is denied completely (often because the whole concept is embarrassing and/or unnecessary). This book rectifies misconceptions on both sides and presents an interesting and well-researched investigation into angels.
Being Catholic, Georges Huber cites the authority of recent popes (being written in 1983, his references go up to John Paul II). Those popes did not just write theological treatises or exhortations about the angels. They actually asked the angels for assistance in concrete ways. Pius XI often asked his guardian angel for help and recommended that others, especially diplomats, invoke their own angels to influence other diplomats through angel-to-angel persuasion. The popes often give credit to their angels for inspiring ideas and giving them energy to continue their difficult work, similar to the way Jesus was comforted in the desert by an angel after the temptations of the devil and He was strengthened by an angel in the Garden of Gethsemane. The author also refers to the many other biblical stories of angels intervening, like in the book of Tobit or the three young men in the fiery furnace (Daniel 3). Church authority and Scriptures agree, angels exist and have a role in history, though often that role is in personal histories and as emissaries from God.
Huber also provides arguments about the existence and activities of angels, using Thomas Aquinas and C. S. Lewis, among others. Humans by nature are able to know and do the good, but their nature is damaged by Original Sin and often they make the wrong choices. It's only natural for God to assign guardians, not just to individuals but even countries and authority roles (like government offices). The good angels offset the work of the bad angels in tempting modern people to do the wrong thing (though we do a good job tempting ourselves). An individual's openness to angelic influence is naturally hard to pin down.
The conclusion of the book is enlightening: "We could have spoken about 'the death of the angels', in the same language as 'the death of God'. In that connections Paul VI said that it is not that the sun has been put out, but that men's eyes have been darkened. God has not died in the way a great figure might die: he has died in the thoughts and in the hearts of many men an women." [p.113] So also angels are denied by those who refuse to look for them. Why not make use of such a fabulous resource?