Works
Ad Petri Cathedram, Aeterna Dei Sapientia, Encyclicals, Grata Recordatio, Mater et Magistra, Meditation for the Feast of St. Francis of Assisi, Meditation for the Feast of the Guardian Angels, Pacem in Terris, Paenitentiam Agere, Princeps Pastorum
Everything2: Pope John XXIII
Biography of the most popular pope of the 20th century.
Illuminating Lives: Pope John XXIII
Essay on the inspiring life of John XXIII, by Beth Randall. Illustrated.
Office of Papal Liturgical Celebrations: Pope John
Biographical article published in L'Osservatore Romano 6 September 2000, and Pope John Paul II's homily at the beatification.
Patron Saints Index: Pope John XXIII
Illustrated profile, with links.
Pope Giovanni XXIII
Official web site of the birthplace of John XXIII, in Sotto il Monte. Lavishly illustrated biography, some episodes in greater detail, news, petitions. All of his encyclicals, and some other writings by or about him, in PDF.
Pope John XXIII
Feature article on Time magazine's Man of the Year for 1962. [Time]
Pope John XXIII (Angelo Roncalli), Bishop
Biography. With prayer in traditional and contemporary language.
The Holy See: Pope John XXIII
http://www.vatican.va/holy_father/john_xxiii/biography/documents/hf_j-xxiii_bio_16071997_biography_e
Short biography of John XXIII, at the Vatican's official web site.