As far as Amazon goes, don't believe the "Customers who bought *this* also bought *this*" routine. Back when my book was an iUniverse title, lo and behold one of these lists popped up for it. Funny thing was the same list popped up for several dozen other iUniverse authors all at the same time...all with the same books on it. It's strictly a marketing ploy, and is not based on any real comparison of actual purchase combinations by customers.
Just to really make my day, Amazon recently opened an "Apparel" online store. So to promote this, they're taking the "Customers who bought" schtick and really running with it. If you look up the current edition of my book on Amazon, on a random basis you may get a truly insipid message on the page that says, "Customers who wear clothes also shop for: 'Clean Underwear' from Amazon's Target Store." I kid you not. Thanks, Amazon, that looks really nice when people are shopping for my book... (Are there customers who don't wear clothes? I suppose if there were, they wouldn't need to shop for clean underwear... or dirty underwear, which one presumes must also be available by the logic of Amazon's sentence...)
At least at the local bookstore you don't have to worry about clothing recommendations...