Were you sleepy?
If you were dropping off as you were reading the book, you might have had a brush with an interesting psychological phenomenon.

As you're going to sleep, or just waking up, there is an experience known as a "waking dream" or "sleep paralysis" where you are still conscious but in a strangely odd state. You tend not to be able to move, and you often have the experience that someone else is in the room with you, even if you can't see them.

This is usually given as a simpler explanation of alien abduction, as well as the old stories of succubi and incubi.
( Amish Information Systems hates misplaced, commas, Thu 3 Jul 2008, 11:53, Reply)