When I was researching Roman Cieslewicz’s posters I looked on Pinterest, which is a website that I started to use recently and it has being very interesting and helpful as its variety of work is huge. Others great aspects about it are the images that the website sends to you weekly based on what interests you, what you see and what you pin, and the number of images that appears related to one single image that you like. In one of my researches about Polish posters I saw a beautiful image of a Polish book cover, which led me to the Pinterest page of the person who posted it, and than led me to a very interesting website, 50watts.com where I found an article with thirty book covers from Poland.
This website is a great source of book-related design and illustration.
I hope you enjoy as much as I did.