Evolution of the Trade Paperback
Back in the day (say, until the early nineties or so), there were really only two types of books you could buy - there were hardcovers and there were mass-market paperbacks. Hardcovers were (and still are) large, ungainly volumes aimed at the early adopters more so than the stereotypical book buyer. They were built to last but ultimately unsuitable for the commuter or vacationer due to their bulk.
Mass-markets, on the other hand, were small, lightweight, ridiculously cheap to print and the epitome of portable. Problem was, they fell apart after three or four reads - their bindings were cheaply glued, they were printed on a substance not unlike newspaper and their pages yellowed after a year in anything close to direct sunlight. They were designed to be disposable.
That was all well and good until book prices started to skyrocket. As the price of mass markets edged closer and closer to ten dollars (it's not quite there yet, but just wait), people started demanding more for their money. The nature of what a book actually is subtly changed - they went from being cheap, throwaway purveyors of information to keepsakes. Reading suddenly became incredibly hip and consumers started to want their books to have a certain permanence while simultaneously wanting them to fit in a purse.
And so the trade paperback was born. They are larger (in cover dimensions) and thinner than their respective mass-markets. The bindings are stiffer, the covers are thicker and the paper is of a much higher quality. They're more expensive to produce (though not THAT much more) and typically cost a bit less than twice as much as their cheaper compatriots.
Sounds like the best of both world, right? Cheap, portable, durable. It was...until the publishing companies started to get greedy.
Mass-markets have that name for a reason - they appeal to the largest sector of the population and (sales taken into account with production costs) make the most money for the publishers. Only certain books make it into mass-market format - romance, sci-fi, mystery and extremely popular authors get published that way because it's pretty much guaranteed that their sales will be low but continual. 70% of the fiction published as paperbacks will stay as trades - the remaining 30% make up roughly 75% of the profits. It occurred to someone after the trade paper revolution was almost completed that, on the whole, the weren't making as much money on their new format as they should have been.
Ever so slowly the permanence of trade paperbacks was traded for an illusion of permanence. A trade paper's individual construction didn't really matter provided it was perceived to be of a higher quality than that of a mass-market. Once consumers became accustomed to ranking book formats as good, better and best instead of in more concrete terms (a feat pulled off by throwing exorbitant amounts of money into cover designs and extremely expensive photorealistic printing methods) the actual printing, binding and construction quality slowly slid downhill.
Nowadays trades don't last all that much longer than mass-markets do - they fall apart in the same way as their little brothers, they just look cooler. More than anything, the rise in popularity of this particular format proves that value has little or nothing to do with cost. Don't be fooled - if you want your books to last, stick to hardcovers.