With 64GB pendrives around, carrying a quarter of a million books in your pocket is no longer a crazy dream.
Pendrives have been shrinking in size and increazing in capacity for quite some time, and for now atleast, 64GB is the biggest you can get.
Just to give you an idea of what you can do with 64GB:
It is enough to store over a 1000 hrs of MP3 audio
You can store almost 250,000 ebooks
You can store over 15 full DVD's i.e. Over 30hrs of DVD video
You can store over 300,000 JPEG compressed images
64GB may seem a little small considering that you can get a portable HDD of over 1TB nowdays. However since this is a pendrive, it fits in your pocket, and doesnt need an external power supply. It will also give much much better data density than an equlvalent sized external HDD.
The pendrive also comes with additional software which allow you to use it for backup. You can also use the pendrive to unlock your PC, and for storing internet passwords.
If you want the highest capacity you can possible get in the form factor of a pendrive, this is you best bet.