Just a point on the 20th ones - they are nice but they have very few pages. We didn't see a lot of characters in June but halfway through our trip we had to buy a second autograph book - just based on our meets during our meals. This is because it has a page for autographs and then opposite a plastic wallet thing for sliding in a photo.
It is nice to put the photos in securely but if you are planning on doing a few character meals or character meets, then I would think about buying a none 20th edition one (the 20th one is the one on the Disney Store link from above) when you get there.
BTW - I was in Poundland the other day and they had some very large Disney biro pens - ideal for characters (particularly costumed ones) to sign with.