For this case, I think “photobook” would be the best choice. If secondary types were a thing, maybe “guide” as well.
I don’t think “type” and “genre” are the same thing. For example, I don’t think “Romantic poetry” should be a different type from just “poetry”, it should be type “poetry” and genre “romantic”. Or a philosophy textbook about Kantianism could be type “philosophy”, genre “Kantianism”?
But, from that philosophy context, there might be smaller works focused on individual parts of some philosopher’s system that people might want to search for. I don’t think “kategorischer Imperativ” is a genre or type even though there are countless books about it. I’m not so sure about this one.