The Goldilocks Dilemma of Reading

I love the online Guardian's book section; there's always something of interest there.  This morning I came across this article/picture gallery of the ten most difficult books to read, chosen by Robert Crum.  I got to thinking...I used to enjoy a "difficult" book much more; I found it challenging and felt very accomplished upon finishing it.  Especially if I had understood it. :-)

Perhaps I read too much now, but lately I get annoyed if I feel a book is difficult just for the sake of being difficult.  Sometimes I'm not in the mood to work that hard.  Yet, if a book is too light or easy, I'm not interested in reading it.  Hmm...the Goldilocks Dilemma of Reading: I have to find a book that's "just right"!

Click here to check out the Guardian piece.  And tell me, what is the most difficult book you've ever read?