I'm not ashamed to say that the copies I own of "Decline and Fall of the Roman Empire," "Life of Johnson," and "Wealth of Nations" are all shortened greatest hits selections rather than the full length originals by Gibbon, Boswell, and Smith. (Here is P.J. O'Rourke on "Why Is The Wealth of Nations So Damn Long?") And I wish I had a shorter version of "Tom Jones," which I tried to reread recently, but gave up with about 700 pages to go.
If I were a high school English teacher, I'd welcome condensed versions of books. They'd be less intimidating to students and they'd take up less time in class, so you can move on to other books. All the economic incentives these days are for publishers to churn out thick books in which readers can wallow in their favorite author's writing, but classrooms contain a wide variety of tastes, so a class is better off with more shorter books than fewer longer books.
With lots of older books, you could just cut out the descriptive prose. Before visual images became hyperabundant, people had a hunger for mental imagery. So, as late as "The Maltese Falcon" in 1930, you have to endure two pages of description of what Sam Spade looks like, which turned out to be not at all like Humphrey Bogart -- Hammett's Spade is 6'3" and blond.And lots of fat books have a thin book lurking inside. For example, Tom Wolfe's 426-page The Right Stuff could furnish a terrific 125-page biography of Chuck Yeager.