Project Gutenberg is a digital library volunteer effort that takes old public domain and converts them into freely available eBooks for the benefit of the general public. Founded back in 1971, the library now has over 42,000 items in its collection.
Among the books in its collection are a number of old books on the subject of photography. One such book is the 1881 title, The Art and Practice of Silver Printing by Capt. Abney and H. P. Robinson (shown above).
A quick search of the Project Gutenberg library for the term ‘photography’ reveals a number of other public domain books — 37, to be exact.
Some of the books feature photographs shot centuries ago — color photos of birds, for example — while others are about the technical aspects of photography.
A search for “camera” also returns a handful of results.
Most of the photography-related stuff you’ll find in the digital library probably won’t be very applicable to the imaging you do nowadays, but if you’re interested in history and old photography techniques, it’s definitely worth spending some time poking around in the collection.