If I get a book with a poor table of contents I'll often correct that and send it to my library as a doc file. Other than that it's simpler to just download from the library.
My reason for 3G isn't about downloading. I'm in the habit of looking things up as I read and when I'm away from the house I like to be able to long press and get the Wikipedia article, which I can do with 3G. With Wifi only I have to wait and then when I get home I rarely remember. This is all part of being a know-it-all.
The truth is lately I've been reading on my phone away from home so I rarely use the 3G now. I'm not sure if that'll continue or not. In any case I have enough 3G devices if it doesn't.
Barry