Without any knowledge of the book or the author beforehand, several weeks ago I picked up ‘The Earth Tripper’ by Leo P. Kelley (1973) based on the title, cover art and back cover blurb. It started out well but bogged down a bit toward the middle before ending as sort of a love story. Not surprisingly the characters’ dialog contained the word “groovy”. I think that one word tells you all you need to know.
More recently, during a visit to a different bookstore, I discovered something that looked very familiar.
I’m curious how often cover art is borrowed or reused.
soylent soup for the soul 
I did a post on reusing cover art a few years back — I know of quite a few other examples but I’d have to dig them up….
http://sciencefictionruminations.wordpress.com/2011/12/25/adventures-in-science-fiction-cover-art-reusing-cover-art/