I suppose I found error in code:
1) Listing4_5 and following, function IsKeyValid(string key):

I think here "return true" and "return false" must be exchanged.

Chapter 9, page 397:
"There is no need to call the Start method for the new Task<string> returned by
the DownloadFileAsync method, because the TaskCompletionSource<string>
instance starts the underlying Task<string>."

must be changed to:

"There is no need to call the Start method for the new Task<string> returned by
the DownloadFileInTask method, because the TaskCompletionSource<string>
instance starts the underlying Task<string>."