Complex sentence: Dark Age Ahead is a 2004 book by Jane Jacobs describing the decay of five key pillars in the US and Canada; this decay threatens to create a dark age unless the trends are reversed.
Split into simpler sentences: Dark Age Ahead is a 2004 book by Jane Jacobs describing what she sees as the decay of five key "pillars" in the US and Canada. She argues that this decay threatens to create a dark age unless the trends are reversed.