I wouldn't consider that a factor in picking storage. They don't take up much space at all.
Really, the only thing i'd want 64gb for is if i'm going to put a lot of HD movies or video on it. With all the streaming options, i probably wouldn't even use it for that.
I agree. Books take surprisingly little space. Music and Movies are the big memory consumers.
Moreover, all of my book readers support a personal bookshelf in the cloud. I have about fifty books on my iPhone with another 250 in the cloud. Retrieving a book from the cloud takes only tens of seconds.
Browsing a real, as opposed to virtual, book store one day, I found a paperback on the shelf that I wanted to read it. I found it in a virtual bookstore, purchased it, and downloaded it while my nephew was standing in line trying to pay for a paperback. True story.
A colleague sent me an announcement of his new book yesterday. I asked him if it was available in electronic form (even one book is too heavy). He responded that he had hoped that it would be available as an e-book on the publication date but that I would have to wait until next week (May 20) for it to show up on Kindle.
One feature of the Kindle reader that I really like is that it keeps me on the same page of the book across all of my devices. I can switch between using my PCs, my iPhone, and my iPad to read a Kindle book. "Kindle in the cloud" (maybe its that little kid sitting in the Kindle tree) always knows what page of the book I am on.