I'd love to hear more about your trip. Please tell us all about it..
My longest solo hike is a very modest five days (unlike the hordes here who gladly saunter into the Alaskan wilderness for weeks at a time with nary more than a tarp, ziplock baggie and a roll of tums) and though my solo trips are generally short, I always bring a book with me, specifically books that do not contain any references to (a) bears eating solo hikers and (b) is paperback.
I try to limit my reading so that I don't blast through the book too fast. On an 18 day hike I would probably bring a couple of paperbacks or perhaps a more weighty tome (2.5 pounds), such as Richard Rhodes' Pulitzer Prize winning "The Making of the Atomic Bomb" which would, besides be the heaviest object in your pack, pretty much explain the early development of atomic energy and the making of the bomb in its 928 outstanding pages.
At a more economical(and backpacking-friendly) 9.6 ounces I would recommend the National Outdoor Book award winning, "The Last Season" by Eric Blehm. It follows the riveting story of a legendary backcountry ranger who disappears in the High Sierra. It's a terrific book that keeps you guessing. I read it on a rainy Sunday (384 pages).
Amazon has it here...
The Last Season
The Making of the Atomic Bomb
If you are not much of a reader, you could always bring an MP3 player and load music or audio books on it. My local library allows you to download audio books to your MP3 player (does not work with iPods, though).
Myself, I prefer the quiet of nature, but to each his own. If I were out there for 18 days, I suspect my tune would change.
Please do tell us more about your trip!