Hello Dave
I rode this route the year before last just using the book, the ACA maps go a different way down through Bremerton. I copied the pages to take with me rather than taking the book and discarded each days details at the end of the day.
The campsites in Washington State are very expensive at about $14 for a hiker/biker site than in Oregon/California where they are about $4. Make sure you have plenty of quarters for showers and laundry. The campsites are plentiful and should be quiet by September.
You won't need to take a GPS unless you really want to, you can't get lost - just keep the Pacific on your right
Are you starting in Vancouver? If yes for supplies go to MEC on Broadway, they are like REI in the US and very helpful.
You can do the whole route without a cooker if you really want to. There is nowhere that you can't get food nearby but I prefer to make a cuppa in the morning.
There are loads of micro breweries on the way down and the local shops offer some lovely beers. Fruit stands also abundant and good value.
Heading into California I was told that it got very hilly but by the time you get there they don't seem that bad. The worst bit of the route has to be from just north of Malibu to the border and if you are not fussed about doing the last 100 miles I would suggest flying out of LA. The airport is accesible from the route.
On an airline note I see that BA are flying direct from San Diego from June. I used them and they were brilliant with the bike.
For hotels in Vancouver I can reccommend the Coast Hotel close to the airport. They pick you up from the airport, it is on the bus route to get to MEC and if you are going North up the Sunshine Coast and across to Vancouver Island the hotel is virtually on the route.
Hope this has helped and if I can help with anything else please let me know.
Lisa