Old Children’s books, free online. Just the covers would have been lovely, but this site hosts each book in its entirety. You can browse or sort the collection according to all sorts of criteria, but the most interesting is to start at the beginning. There are about 2,500 books though, so it might take you a while. (And here’s a similar site, though much smaller.)
Only the covers are available on this site, but what a treat they are, especially if you’re into graphic design: Children’s Books of The Early Soviet Era
More old books and other written media from The Netherlands at The Memory Of The Netherlands. There are also photos, paintings, film, posters, artifacts, and so many other things on this site that you could amuse yourself for days, browsing through them. The site is in English too!
These days books have websites, and even trailers devoted to them, but not so long ago it was all so very different.
The post-nuclear-apocalyptic Z For Zachariah scared the bejeebus out of me when we read it at school and it’s a book I still remember quite vividly. Looking for information on it, I found this site: Lost Books. Reviews of forgotten and underappreciated Sci-Fi and Speculative fiction.