Well, so far I haven't actually been very caustic. How about this then? The following is one of the worst book covers I own--or have even seen.
No one gets hurt? My eyes, my eyes! Lurid and off-putting, this is the cover for Russell James's thriller, published by the Do Not Press in 2003. It is now, perhaps unsurprisngly, out of print.
To be fair, the book itself may well be great. I read a review of it when it came out, and was intrigued enough to order it in at a bookshop here in town. It was not until it actually came in that I saw the awesome hideousness of the cover. The chap behind the bookshop counter also saw it then for the first time. It wasn't really possible not to buy the book--having special ordered it from the UK, it would not have been possible for the bookshop to send it back.
It's quite weird, because the cover is actually quite appropriate, in its own appalling way. The book's heroine "enters the murky world of call-girls, porn and Internet sex [... and] she discovers that she is pregnant. Despite his unconvincing denials, [she] is shocked to discover that the father of her unborn child is involved with the pornographers." So, yes, the grinning, gormless nude blonde on the DVD cover and the pregnancy kit are all relevant to the plot, but lawks, if that ain't the least appealing cover they could have created.
I suppose this cover would catch your eye if you saw it on a bookshop shelf, but would you buy it? Would you be able to hand that over to the person behind the bookshop counter (especially if it was a woman)? Would you be able to read it in public? No wonder the book went out of print--people must have avoided it like the plague. I've had it for 4 years now, and still haven't been able to read it (I do a lot of reading on the bus, and I don't really want people thinking I'm leafing through a catalogue of pornographic DVDs).