A passporter is a guide book for WDW. Besides the guide book, it has 'pouches' in the back for each day of your trip - invaluable for storing receipts, tickets, maps, etc. Each pouch also had a mini-journal for each day, where you could record your impressions, and document how the day went. It also has some planning worksheets, budget sheets, etc.
I have had both the Deluxe Edition, as well as the Standard Edition. No difference, really, except for the price. So, I think it is a personal decision on how much you want to spend.
As far as how to find one, our local library has older versions (of course, you won't be able to write on the back sheets!), but you could check it out and see if it is worthwhile. If you decide to purchase, you can find them at most major bookstores, as well as
Amazon.com, or directly from Passporter, at
www.passporter.com.
Their website also has alot of planning tips, reviews, etc.
Hope this information helps.