Your local bookseller wants to move merchandise off the shelves of your local store. Yes, he'lll sell you a cup of coffee while you give the book a once-over, but he's not running a library. Therefore, what you see or do not see on your local shelves is much more an indication of what's moving in your local store than of the health of the language in-general. Any retailer watches carefully to see what is selling where, both to determine how many units to order (and how many unsold units to return to the publisher for pulping), but also to transfer product from one store where it isn't selling, to another store where it is. Technical books will be found in greater abundance at stores which are close to Universities and office-centers where geeks work. Computer books, in paper form anyway, are notoriously expensive, slow-moving and hard to sell. Just look at how many books which have the misfortune of having the once-necessary word “2007” on the cover which are now selling for 15% of the cover price if that. (Next stop: to become newsprint or a grocery-bag.)
Knowing this, the good Mr. O'Reilley actually sells most of his material on-line, for electronic delivery, which of course makes a great deal of sense to him since they're all just DocBook files anyway. His service can spit out a PDF or whatever-you-want, produced on-the-fly, and either delivered to you or simply made available for online browsing. I'm quite sure that those electronic-book readers are going to be very big in our industry... and good for the planet, too. Ones-and-zeroes weigh nothing.
As we all know, “fashion sells.” Last year's trick is last year's trick. There are literally billions of lines of COBOL out there doing heavy-lifting every microsecond of every day, but books on COBOL are best found at MacKay's (Used Books). The books that are most likely to be placed in retail bookstores are fat (at least 1.5 inches on the edge), cheaply printed with wide margins and big type (to make them fatter...), splashed with easily recognizable trademarks, and are written about “fashionable” subjects like (at the moment...) Ruby.
Here's a wee thought for you. Look at yourself, as a bookseller would. Right now, you are probably feeling that there is something you don't know. Therefore, you are probably fairly self-motivated to buy a book about it soon, if you haven't done so already. Okay, then, what kind of book are you most likely to buy? One about Perl? Probably not. One about Ruby or Haskell or some other “shiny new thing” that promises to be the next silver bullet? Probably. The book that is most likely to make it to the checkout is one that will be written with the slant that you probably know one or more similar programming languages, you want to find a CD-ROM or DVD-ROM in the back cover (although that's expensive), and you'll want to “kick the tires a bit” with some easy-to-do projects that you may or may not do. You'll spend about a week with the book, and put it on your shelf for reference, or you might (bad dog! cheap skate! no biscuit!) return it. And that is what a good retail merchandising director makes it his or her business to know!