I think he's completely wrong about school texts. The cost of textbooks in college is already driving students to bootleg digital copies of textbooks and pass them around like really fat MP3 files, and public school systems are looking at it as an alternative to being screwed with by the textbook publishers, who for many years have been packaging unsalted oatmeal as Oatmeal Squares and pricing it accordingly. I think the recent decision of the Texas state school board regarding history textbooks is going to be a spur to this. So I think the current model of textbooks in schools is going away over the next couple of decades. Netbooks are getting insanely cheap, used PCs can be had for landfill prices, and money-starved school districts are going to jump at the chance to use digital texts that can be locally modified/annotated.
Comics, though - he's got that nailed. American comics are increasingly locked into a "collector" model and are no longer pitched to the teenage boys they one were aimed at. They're intended for guys in their 20s and 30s who want edgy graphic novels they can read once and lock up in plastic bags as an investment, and they're getting their lunch eaten by manga, which unlike comics aren't just about spandex-clad superheroes. Add in the inability of DC and Marvel to come up with any appealing new characters since the 1960s, the increasing popularity of movies built on the old characters, and you can understand why Marvel sold out to Hollywood. That's where the money is in comics now, changing the heroes of yesteryear's comics into movie stars. Granted, this has been going on with manga and anime since the 1950s, but oddly enough it's been the Japanese who have shown more creativity with both print and video media in this regard.
Magazines are more of a mixed field. I think the magazines that remain standing are going to either cater to a niche market that can sustain the print advertising model, or go digital, or remain alive as fusions of print and digital formats. We're already seeing the "major" newsweeklies like Time and Newsweek struggle with profitability in the same way their parent media conglomerates do, while the third national news magazine, U.S. News & World Report, has become a monthly magazine with most of its news content shifted to its website. I suspect that the future of news magazines is going to become pretty grim as they compete with newspapers for eyeballs. Either they'll have to offer more in-depth reporting from better writers, or they're going to continue losing market share. Perhaps the fate of the newsweeklies is to become opinion centers like National Review and The Nation, both of which supplement their dead-tree editions with extensive on-line websites. Literary magazines have largely vanished except as niche/genre publications; even the venerable Reader's Digest has gone through bankruptcy, and the Saturday Evening Post survives only as a quarterly published by a 501(c)(3) non-profit. Even the once-numerous science-fiction pulp magazines have dwindled to a handful of survivors, although online magazines seem to be multiplying and providing new markets for aspiring writers.
Novels, yeah. Those are going to survive in paperback and hardback and the increasingly popular digital formats, but I think individual sales are going to drop off - with the fracturing of American mass culture, there won't be so many blockbuster best-sellers any more, but the Internet is going to allow more mid-list authors to make some money thanks to the Long Tail. Used books are also going to be popular, because not everybody can afford a hardback at $30 or a paperback at $9, and the internet makes it easier to find used books - not just through Amazon but through dedicated online used book marketplaces such as abebooks.com, alibris, and Powell's. You may not see used book stores in big cities any more, since real estate in those cities is expensive, but out in the burbs and small towns where storefronts and warehouse space are cheap, UPS and the USPS (while it lasts) will keep used book sellers in business - to say nothing of SF conventions, which support a small population of genre booksellers who deal in used, new, and collectible books. I don't think Cobb's prediction of a Great Digitizing is going to happen - authors will fight tooth and nail against the conversion (theft is almost but not quite appropriate here) of their intellectual property, for unlike musicians, they can't go on concert tours to make up lost revenue from e-books shared on p2p networks. Also, digitizing and proofing all those pages isn't cheap - somebody's got to be paid for all that work, and somebody has to pay for the scanners and space...it all adds up. Lots of people don't care to read books on the computer or the Kindle, so I see a lot of dead trees in the future along with all the e-books, webzines, and what have you.
Collectible books? Also yes. People have always shelled out for first editions, special editions autographed by the author, and omnibus collections. That won't change, and we might even see more of it. One thing's for sure, the next twenty years are going to be very interesting in the world of publishing.