David Horowitz, red-diaper baby, knows whereof he speaks when it comes to the Left.  His books are essential reading for understanding the mentality of leftists.  His latest, Radicals: Portraits of a Destructive Passion, is reviewed here.

I recommend all of Horowitz's books.  Radical Son, though not quite at the level of Whittaker Chambers' Witness, comes close.

You haven't read Witness?  Then get to it!  It is a book of high literary merit that delivers crucial insights into the human predicament.