Joining WKA now would be fine. You’ll likely get the “new” tech manual in Nov or Dec anyway for the 2014 season which starts in Dec 2013 at Daytona. You could request a 2013 tech manual and go through it while you wait for the new one. And besides the changes from year to year aren’t that big anyway. Especially for someone new who just needs to get an idea of what’s going on.
Give the WKA office a call. You might be able to request the manual while waiting for your membership stuff to arrive.