That's all very nice in theory, but if I am not a domain expert in a subject and the book's cover is made of clip-art and Comic Sans, human nature will lead me

That's all very nice in theory, but if I am not a domain expert in a subject and the book's cover is made of clip-art and Comic Sans, human nature will lead me to trust it less. Humans are not rational animals, no matter how much we'd like it otherwise. Rather than shouting down the tide, it's wise to invest in a little design/marketing - even if it rankles.