I think books will slowly become less common but will still be ever-present. But that might just be an opinion based on the inability to imagine a world without books.

I don't think so. Not everyone likes using e-books. Not only can it strain eyes at times, but there's a special feeling about having the physical book over an e-book. I think it'll stay about the same as it is now.

Oh, I doubt books will disappear. Books and ereaders perform different functions and fill different needs.
I don't agree with everyone on a tear about ereaders. (I myself am philosophically opposed to them, but only philosophically. ) They're infinitely handier than carrying a duffel of books on every trip, they're nigh indestructible (or at least, Kindles are; I rolled an F-250 over mine and then left it in two feet of snow in 9* temperatures for two hours, and then my father stepped on it, and it still works fine), and I can read on them without getting carsick, as I do when reading paper.
However, they will not replace books.
Books provide a tactile and aesthetic experience that most readers seem to value in itself. Ereaders cannot offer the feel of the paper, or the look of the cover on the coffee table, or a vast collection of spines on a shelf. It's much harder to jot notes in your margins, and the current bookmarking system is clumsy and difficult to navigate. (I like sticky notes better.)

And you know, we've had cars for quite a while, but bicycles haven't disappeared. Word processing programmes haven't made it more difficult to buy spiral notebooks. MIDI keyboards haven't replaced woodwind or brass or string instruments. Rolly chairs are worlds of fun, but no one furnishes their dining rooms with them.

Now, for someone only interested in absorbing a given set of words, I can see that individual choosing an ereader and never going back, but by and large, ereaders and books just belong in different niches, and thus do not threaten one another.

Very interesting query.

Well, I am an old-fashioned physical book aficionado. I chose "phys. books will remain as common as they are now" as my answer. Why? Here is my opinion:

eBooks: -Require a cumbersome electronic device to read them, which effectively becomes an ugly paperweight when lacking a power source.
-Are not exactly coffee table material.
-Feeds a cultural trend bent on frivolous technical gadgetry for it's own sake.
-If device breaks, malfunctions, you're out of luck.

Physical books: -In order to read one, all you need are a pair of eyes and a light source, be it electric, sun or candles.
-The only thing that can really break is the spine. Yes, pages can be lost, soaked, torn or defaced. But if you soak a Kindle, I bet you're also out of luck.
-As time goes on, books will be increasingly seen as valuable forms of media. For historical value; for price-conscious value; for longevity value (we still have books from the 1800s in old bookstores in good condition.) I don't think a two-hundred year old Kindle will work. Nor will it be a collector's edition.
-Hi there! Could you sign my Kindle? hehehe.

People like me, enjoy reading physical books and magazines. Give me my old National Geographic with the gorgeous Kodachrome vintage pictures and the ads for land yachts. Or an old, hardbound Wizard of Oz. I love real books.

Skipping all the sentimental arguments, we'll run out of trees eventually.

Novel commodity? I see what you did there

I think books will slowly, gradually become less common place - but I still think we'll have them in a hundred years. Books serve a purpose and tick boxes which e-books don't really cater for. The visual presentation, tactile sensation, low-tech (and highly reliable) nature all make them valuable. There's no vendor lock-in, no digital tracking or control. You can write on a book, share it, use it any time, or have it open while you work.

It'll take a few generations for things to really swing in favour of e-books - but I think there will become a time when the printed word begins to feel wasteful, inefficient, archaic and bulky. The youngest generation now will still learn from books and use them often in their lifetime - but perhaps the next-but-one will have less use for them.

I don't think there will be a strong relationship between the rise of the e-book and the fall of the physical book. E-books will get bigger and bigger quickly - while books will drop off slowly. The price-point (and, arguably, quality level) of e-books means they'll be much more disposable.

On that note, assuming the technology remains stable, I'd expect magazines and newspapers to fade out pretty quickly in the next 5-10 years. What value does a newspaper have a week after its publication? A month? A year? These transient media are much more suited to digital formats - especially in this waste-conscious age. But books are built to last, to be looked after and treasured. They'll be around for a long time.

It's interesting which technologies become obsolete when superseded and which find a new niche. Telegrams are gone, but radio's sure hung around. Cassette tapes gone, CDs going, and then Vinyl makes a bit of a comeback. Energy efficient induction cooktops and old school charcoal barbeques/grills seem to coexist nicely. Plus email and IM has almost added to the value and appeal of genuine, hand-written snailmail.
E-books are pretty cheap and storage-efficient... but just aren't the same, sentimentally. I find with books read on kindle I'm less likely to recall the author or even book name for example, possibly because I'm not constantly looking at the cover/side each time I go to read it. Also you can't really lend an e-book or give one as a gift, have a collection sitting for display on your bookshelf or coffee table. I predict in future a kind of dual-tier where physical book sales indication a degree of prestige for the authors.

_{No, physical books will remain as common as they are now because people like me love them so.}