I have removed bad links from my Chesterton Page and have added literally dozens of online books (many, but not all, from Internet Archive editions), so that there are more than 80 Chesterton books by or about the great apologist and literary and social critic now available to read at no charge: practically every book written by him, and certainly all the more famous theological ones.
This listing is, without doubt, the most complete on the Internet in one place of Chesterton's full-length books, available online, and features a chronological listing too. Many titles were not listed even on Martin Ward's extensive listing of GKC books and articles. I always selected html when it was available for format, with the second choice being txt. Some of the book files (many of the "alternates") are PDFs. Additionally, there are a few audio books, culled from Martin Ward's page.
I've managed (as a special "bonus") to find some relatively rare photographs of Chesterton (several I had never seen): a page with eleven of them. One even features a broad, open-mouthed smile! Here's another copyrighted one I don't recall ever seeing. Enjoy!