The Catholic Encyclopedia was an early 20th century "international work of reference on the constitution, doctrine, discipline, and history of the Catholic Church". It ran to 16 volumes (with index), plus later supplements.
The first edtion was published from 1905-1912 by the Robert Appleton Company. A later "special edition under the auspices of the Knights of Columbus Catholic Truth Committee" was published 1913-1914, with substantially the same content, but with some new material added. Supplementary volumes were published in later years.
A hyperlinked, transcribed edition of the Catholic Encyclopedia is available on the New Advent website. While based on the first edition, it does have some updates reflecting more recent developments (like the canonization of certain saints).
Page images of the complete 1913-1914 edition, along with the 1922 supplement (as volume 17) are available at HathiTrust.
Links below go to digitization of individual volumes, from the Internet Archive and other sites.
This is a "meta-book", which stitches together separate files elsewhere on the Web as they appeared in a previously published book. It is subject to removal if someone produces an integrated edition. If that happens, The Online Books Page will point to the integrated version.