Phil, no need to book, I just turned up and paid on the day. Its a park and ride from the local airfield so you park there and they bus you in. It does cost to park though, but can't remember how much, something like $10 -15 though. Amelia is a nice day out, some stunning machinery on show.
If you need anymore info, don't forget that Rick Ostman from turn 10 works the show every year, as one of the organisers.