Increasingly books that are in the public domain are being digitised and made available online. Two sites for free e-books are:
Public library websites often provide access to e-book collections for borrowing as well as links to sites providing free e-books for download to add to your collection.
Typing the title of a book AND e-book in a search engine (especially if the title was published prior to 1955) is one way of discovering the availability of titles.
In 1839 George Mackillop submitted an article to the Scottish publication, Quarterly Journal of Agriculture, describing his views about the possibilities of the new settlement of Port Phillip. I was unable to locate a copy of the article until I found a copy of the volume containing the article digitised by Google.