Have you ever searched for something on Amazon and found that it comes in several different flavors? A couple of obvious examples are this search for "Harry Potter" and this one for "Windows Vista". In the first case, everyone not living under a rock knows there are six (and soon to be seven) different volumes of Harry Potter titles while most folks probably realize the second example is going to kick up multiple versions of Microsoft's latest operating system.
Rather than returning all of these versions scattered across the search results, why doesn't Amazon group them together horizontally? In the Harry Potter example all seven of the individual titles might be the first thing that appears in the search results and they'd all be next to each other, left to right across the page (as opposed to showing up as numbers 1, 4, 5, 7, 8, 12 and 28 in the search results.
You might note that just about every item in Amazon's search results has a description that already occupies almost all the available horizontal space, so how could you fit more than one item in a row? First of all, they'd need to stack the description a little differently to make better use of the room; for example, placing the author name below the title rather than next to it. More importantly, they'd need to introduce the use of horizontal scroll bars so that the contents for that row/frame could be easily shifted left/right, independently of the rest of the page.
Would this require a fair amount of programming and interface adjustment? You bet. Would it create a better user experience with less confusing search results? I think so. It would also require Amazon (or the publishers) to provide the metadata required to identify all the family relationships. Amazon is a data beast though, so adding one more field for every product record won't even cause them to break a sweat.
And before you think that this problem is limited to oddball products like the two I've listed, consider how many multi-volume products there are out there, not to mention all the books that are available in both hardcover and paperback. I think this enhancement would come into play on a fairly regular basis.