Catholic Encyclopedia: Angels article by Hugh Pope, from the 1911 Catholic Encyclopedia. This is an extensive article, divided into subsections, notable among which are hierarchical organizations (the scholastics were very into Ps-Dionysius), angels in the Septuagint, and angels in the Zend-Avesta. The Catholic Encyclopedia has a lot of topics on angels, but is not cross referenced well. Some highlights include Guardian Angels, Early Christian Representations of Angels, St. Michael .
Raphael.net provides various angel resources and texts, including councils and papal pronouncements.
"Angel Popularity on the Rise" by John A. Hardon (a Jesuit) commenting on New Age, Protestant and Catholic angelology. This long article includes an interesting response to two Protestant angel booksBilly Graham's Angels and Ron Rhodes' Angels Among Us. Hardon characterizes Graham's book as "beautiful," but traces Graham's unwillingness to approve prayers to angels as stemming from the supposition of predestination. He dismisses Rhodes' belief that prayer to angels is "Celestial Quackery." Hardon is optimistic about the angel craze, and gives Catholicism some of the credit:
"It is heartening to see the popularity of the angels rising in countries like our own. No doubt one reason for this phenomenon is the influence of Catholic teaching on our separated brethren."
Les Anges Guardiens, a large French site devoted to (and venerating) angels and guardian angels, produced by the Clercs de St-Viateur, Lyons, France. Includes FAQs, first-hand accounts (ancient to modern), biblical references, church texts, quotes, articles, prayers, poetry, and an excellent gallery. This is a wonderfully full site, the likes of which has not yet been done in English. (The site you are looking at is more comprehensive in some ways, but most of the content is external.) If you read French slowly, as I do, you may want to use the translation provided belowand it's fun to see the "Bubble of the Pope Leon X" (Bulle du Pape Léon X).
Opus Sanctorum Angelorum. Catholic organization "aims at fostering an intimate collaboration with the holy angels for the glorification of God and for the sanctification… of souls."
Catholic angel books
Amazon. Angels and Demons: What Do We Really Know About Them? by Peter J. Kreeft.
Editor's opinion: This was my introduction to Kreeft, and I had mixed feelings. He does the sort of friendly but not entirely unintellectual popularizing that C. S. Lewis did. (Kreeft reveres Lewis; in fact, he teaches courses on him at Boston College.) But Kreeft's work is pitched lower than Lewis'he often seems to Lewis what Lewis is to "real" theology. And Lewis has greater charm and literary gifts. At the same time, there's intellectual content in this book, in contrast to run-down of biblical proof-texts and encounter stories found elsewhere.
Excerpt on "Things to Know About Them" and other topics.
List of Catholic Angel books. Author notes recent spate of books are generally "new age or otherwise heterodox."
Amazon. Angels and Their Mission: According to the Fathers of the Church by Jean Danielou, translated by David Heimann.
Amazon. St. Michael and the Angels: A Month With St. Michael and the Holy Angels "Compiled from Approved Sources" (apparently mostly saints' lives). One Amazon reviewer calls it "perhaps the most popular book ever published on angels." Amazon's ranking (359,000) does not back him up.
Aquinas on Angels
Thomas Aquinas, The Summa Theologica on angels (1947 edition, translated by the Fathers of the English Dominican Province).
A poor set of Catholic FAQs, from Catholic Online. The text is dumbed-down, and contains inaccuracies (and even proofreading errors!). It is not true that the New Testament mentions "seven orders" of angels (Angels, Powers, Principalities, Dominions, Thrones and Archangels). This comes from the Christian mystic Ps-Dionysius.