I experienced similar behavior as described in errata #48975. I too was able to get the code “working” by adding @DRb.thread.join@ to the server irb session. The Hello World example worked as is without readline issues, but for this Reminder example I had to pass the @–noreadline@ option to both irb sessions. After that it worked fine for me (without the need for explicit threading) for both 1.8.7 and 1.9.2.
Thanks. Enjoying the book.