Ah I see! Well you'd have two options:
1. An evaluation version of Reaper 2.5 is included on the CD that comes with the book. You'd need to check with Justin I guess, but I am assuming that if you have a Reaper 3 license then that should probably also cover you for earlier versions. You can install it on your computer alongside Reaper 3.x (just use a different directory name).
.. or ..
2. The most basic features are almost all the same in Reaper 3.x as in 2.x - the differences are mostly that extra more advanced stuff has been added. You should be able to work thru the material in the book in Reaper 3 just about the same as you can in 2.x.
Hope this helps and good luck!