What is the purpose of 'Thread.sleep(1000)' in the snippet illustrating a
utility method 'actor'?

scala> val seriousActor2 = actor {
         for (i <- 1 to 5)
           println("That is the question.")
         Thread.sleep(1000)
       }

The code indeed prints out the string 5 times, but it happens all at
once, followed by, rather unnoticeable, 1s pause.

Missing "to" in the last sentence of the page:

  "If you want your program be as efficient as possible, ..."

Highlighted section, in the middle of the page:

  "... programs will need at least a few receive’s, but you ..."
                                                ^
That should've been ordinary plural, "receives".

On Listing 32.6, comment on getIp refers to Listing 30.3 but should
probably refer to Listing 32.3 instead.

(30 seems to have been the chapter number for actors in 1st edition)