How many GB do I need

It will probably serve your needs quite, depending on how often you are willing to synch in order to change out content. However, there is no way to upgrade, so you might want to consider springing for the extra GB. I know I wish I had (although not a huge issue).

eBooks don't take up much space (most of mine are under 2 meg each). The things which tend to take up a lot of space are music and films (I have one 50 min HD program that is about 1.4 gig), and photos. Also some of the 16 gig is taken up with iOS - so (I think) that you have about 14 gig available for your content (I havn't got the 16 gig version so I'm not sure of the actual amount). If you can afford go for the next size up - it's best to have too much capacity than too little, and it can't be increased once you've bought it.