There is a real danger when liberals read the contents of conservative books, they waver in their convictions, they question their beliefs and horror of horrors some of them abandon liberalism all together.
Media Matters had that problem in the 90's when they first started to assigning personnel to monitor Rush.
After a month or so, sometimes even less, the monitors were turned into full fledged conservatives and quit.
They now monitor the monitors and rotate them frequently after only a few segments of a Rush broadcast. The monitors are then deeply immersed back into a liberal environment of watching MSNBC or listening to the soothing voices of NPR.