I'm sure a lot of people find some TPL really difficult to grasp, but then again I've personally found that some people have difficulty grasping what the "if" statement does in C. I personally have difficulty understanding the advanced concepts of Topology,
while there others who take to it like a fish in the water. Simple is subjective.
The updated CLR book should be pretty awesome the first one is well worth a read.
Exoteric -would recommend the LINQ in action book (havent read the apress one)
Jon Skeet is probably nearly done with his next edition of C# in depth which I am looking forward to.