Reading Two Books At Once

In the past I've always dedicated myself to reading one book at a time, with only a few exceptions. Recently these exceptions seem to have become more and more often. The book I'm reading will be too big to take on holiday, or too complex to dip into for a few minutes break, or too likely to give me nightmares to read at bedtime. I don't want to read two books at once, but yet it is happening.

What's your view? Do you read two books (or more) at once? Do you want to?