So for me there's a bit of a difference in book covers and book content. Although I love physical books and am often drawn to a book because of it's cover, I'm just as likely to buy or borrow a book because of the story. The two are not necessarily dependent on one another. I read quite a bit by recommendation so the cover doesn't always make as much of a difference to me.
That being said I do love a beautiful book cover, just as much as I love a beautiful picture or painting. And when it comes to pictures and paintings, I love vibrant colors, patterns, and beautiful typography. So book covers I would turn into posters would of course likely have those characteristics. I've included a few of them below.
I LOVE the type on this one!
This is truly beautiful work! It's actually a journal, but the pattern is so gorgeous!
I HATE the type in this one but it is appropriately cheesy as is the type for most sci-fi novels... The picture, however, I think is gorgeous. This cover made me what to read what's inside. And having read the book, the headdress she's wearing is unbelievably detailed and completely relevant to the story.
Love the colored background with the silhouette here. Very secret-like.
I didn't finish reading this but I love this bright pink cover and the illustration here. The pattern here is just lovely!