I'm really starting to like small books. Yes, I recently wrote a book that's 320 pages long, but in my own reading I've taken to books of under 200 (and preferably 100) pages. Oxford's series of Very Short Introductions is a good example. Another might be the original 1855, 92-page edition of Whitman's Leaves of Grass, which was so short that he said it "would tend to induce people to take me along with them and read me in the open air: I am nearly always successful with the reader in the open air." Others I like: Yevgeny Zamyatin's We, Ayn Rand's Anthem, the fragments of Epicurus (which I'm still working to translate), and Frederic Bastiat's The Law.
Too, I suspect that short books might be the only ones to survive the coming dominance of the e-reader, because you can always squeeze a properly-configured short book in your pocket. If I ever get around to starting a public domain press, the printed (as opposed to online) books will all be highly portable, perhaps even tiny (I rather like the thirtytwomo size: 5.5 x 3.5 inches).