1. The HitchHiker's Guide to the Galaxy
Great! Read them all. Actually, this was what got me started on ebooks. I wanted to know what people were talking about (there was a time when a lot of quotes, names and anectodes my friends made were being told from this book). And since I wanted it in the original language (english), I did some googling and found my first pirated copy of a book real quick.
That was so impressive that I have not purchased another pBook for myself ever since.
Another "topX" list ... I like 'em