If you're going to include trilogies, tetrologies, etc. as long books...then, yeah, books can be very long. And often very boring.
I don't worry about all that.
I do, however, like to alternate long books with short story collections (and my definition of a long book is not one with endless sequels).
Here is a brief list of what I consider to be long books:
1) Miss Macintosh, My Darling - Marguerite Young
2) JR - William Gaddis
3) Underworld - Don DeLillo
4) The Recognitions - William Gaddis
5) Atlas Shrugged - Ayn Ran
6) Parallel Stories - Péter Nádas
7) Infinite Jest - David Foster Wallace
...to name a few.
Not only will I alternate long novels (and even very ordinary-length novels) with short story collections, but I also like to alternate serious works of Literature (as above) with 'trashy' fun-filled, fun-to-read genre novels. A very short list, to give an example, is listed below:
1) Clive Barker
2) Most indy writers (99.99999999999999999999999%)
3) John Brunner
4) Robert Silverberg
5) Ramsey Campbell
6) Brian Lumley
7) Agatha Christie
8) Stephen King
9) Anne Rice
10) The remaining percentage of indy writers
11) John D. MacDonald
12) Brett Halliday
It should be mentioned that #2 and #10 will sometimes be a form of unrelenting torture, depending upon the skill of the writer to construct sentences that make sense, along with (of course) an ability to proofread and edit.
Still, the 12 above are fun to read - torried and unashamedly bad - but each to his own, aeh?
But do try to alternate the novels with short story collections, and the fun with the serious.
Don






