Paradox time: It's far harder to write a good short book than a good long one.
For my rule of thumb about why, I defer to Martin Scorsese, who once said that cinema was a matter of what was in the frame and what was out of it. Selectivity is what makes art, not inclusiveness. Stories are defined as much by what they leave out, where they pick up and leave off, and what they elect not to elaborate on, as they are by what they do contain.
For a not-too-long book that's also good (if I dare say so), check out my (new!) novel Welcome To The Fold, and showing your support for it by registering at Inkshares and adding the book to your "Follow" list! Failing that, you can always buy one of my existing books, available on Amazon Kindle and in dead-tree format.
It can be hard to believe this in the face of the likes of Don DeLillo's White Noise, or David Foster Wallace's Infinite Jest, or any number of other "maximalist" (I guess that's the term?) works from the last couple of decades becoming both strong sellers and models for other authors to follow. Hanya Yanagihara's A Little Life also comes to mind. Maybe I would have been better off picking a maximalist work that I actually liked — Alexandre Dumas's The Count of Monte Cristo, for instance — than one of those baggy bloaters.
I've entertained a couple of theories about how Big Fat Novels tend to slurp up the gravitas gravy. Books are not cheap, so if you spend $25 on something and get several weeks' worth of reading out of it, that's a better deal than $15 for something you'll finish in three days. The word processor has made it surpassingly easy to become a logorrheac, far more so than the typewriter ever did. It's easy to fill pages with convolution and movement and the illusion of depth.
And maybe that's just it: people are more readily moved by the appearance of profundity or complexity than the actually profound or complex thing, and size and heft are a cheap way to provide said appearances. If you write a long, elaborate book, and you deliberately shed yourself of the discipline to find framing and focus for the story, and you're rewarded for doing so by peers or by commercial success, you now have one less incentive to think that (in the words of Milton Glaser) "just enough is more."
People mistake the artifacts of a thing with the actual qualities of a thing. If they see a long book, they'll assume the author was ambitious and hard-working, and be impressed superficially by that. But not all ambitious works have their ambitions reflected in the length of the finished product. Stan Brakhage sweated for months at a time on movies that were barely seconds long, hand-painting each frame of the film. There's more ambition in all nine seconds of "Eye Myth", a year's labor for Brakhage, than there is in any dozen other movies a hundred times its length.
This is a pedantic and tiresome point, and I know it, but I don't think we confront it often enough. Readers see a long book, and are more inclined to go rummaging around inside it for Meaning and Significance, because we assume (not always incorrectly) that the sheer length of the thing is a better habitat for Meaning and Significance than something a fraction of its size. We look at all those pages and we mystify ourselves into thinking they have to add up to something. And, worse, we don't disabuse newly minted authors of this mystification; we allow them to expend time and energy thinking the point of this whole thing is to one-up the likes of DeLillo, or Foster, or William Gaddis, or the great-granddaddy of that whole approach, James Joyce by way of Ulysses (instead of "The Dead").
I think my point is less that these are bad books, or that these are bad authors, and more that they are limited and reductionist examples to aspire to. Their length isn't even really the issue; it's the bad example they set. Or, rather, the way their bad examples are too often not challenged, not addressed as counterpoint or by counterexample, by authors in the shadow of those books, whether or not they seem conscious of being there. The more people are taught by example that the only "serious" books worth writing are in this vein, the less likely we see any other kind of ambition.
Some of my favorite books are so short you could read them over the course of a slow Sunday. Yasushi Inoue's Tun-huang, Osamu Dazai's No Longer Human. More familiar titles abound: Gatsby, Notes From Underground, Zamyatin's WE, I could go on. And there's worlds more in any one of them than there are in any ten other books four times their size.