I mean, really I will not buy a book that has an obnoxious, profane, inappropriate cover. I also tend to let things pass while everyone is telling me that I must read it. If it is a recommended book and I really enjoy it, I am so grateful to have had someone share about its 'inner goodness' with me, despite the cover.
99% of the time that I am purchasing a book these days, I am buying for a specific person, purpose or price. Cover will not be in the top three considerations unless I am leisurely shopping for books (What?!? People have time for this? I wish) and since I have so many unread books, the cover is not a priority for me.
I will admit that I used to shop this way. Ain't no shame in that game, but if this is the only way I chose books I would have missed out on some greats like Eat, Pray, Love, Anna Karenina and The Bible.
Don't limit yourself to the cover - back or front - read some reviews, ask for input or just take a risk and read.
I do not have a favorite book based upon its cover. What has always stuck with me is the book's content. The cover has fallen to becoming a descriptor or an afterthought rather than a determinant of my enjoyment or appreciation of the book.