All Good All Free / Learning for Nothing
Works out of copyright and the strange pleasure of old text
A vast body of published human work sits in the public domain, and terms vary by place.
Once copyright expires, a work generally enters the public domain, which is the legal way of saying it now belongs to everybody. The category is enormous: centuries of literature, philosophy, science, poetry, drama, art, maps, photographs, sheet music, cookery, natural history illustration and reference material. Much of it has been digitised and is freely readable and downloadable. An important caveat: the rules on duration and on what applies to which work differ between countries, and some editions, translations, scans or recordings carry their own separate terms. Check the terms attached to the specific item rather than assuming, and if the use matters, get proper advice.
The material rewards a particular kind of reading. Old technical writing is often startlingly clear because the author expected an intelligent reader with no background, which is a description of most of us most of the time. Old illustration is frequently better than modern equivalents, because someone drew it slowly by hand while looking closely at the thing. Old reference books preserve knowledge that later editions dropped for space. And primary documents, letters, records, contemporary accounts, offer the sensation of hearing directly rather than through several layers of summary.
Alongside expired copyright sits a separate and growing body of work released deliberately under open terms by living creators: photographs, illustrations, code, fonts, teaching material, music, research. These are not the same thing as public domain. They come with conditions, commonly around attribution and sometimes around whether you may use them commercially or alter them. Those conditions are usually short and plainly written, and they are worth reading before use, because the whole arrangement depends on people honouring terms they were given for nothing. Again, if the stakes are real, take proper advice rather than a magazine's word for it.