Finished my first e-book the other day, "Around the World in 80 Days." It was a free download. I want to rewatch the David Niven movie now. IIRC, it follows pretty closely.
Just started "The Island of Dr. Moreau." Also a free download.
I'm about to buy my first non-free e-book. When I finish "Salt" I'm going to get Krauss' "A Universe from Nothing." E-book is only $3 less than the paper book - so it's maybe about $12 instead of $15. I'm kinda surprised it would cost more than $5. Sure, there's extra editing to produce an e-book that displays nicely, but you're not paying for paper, shipping, etc.
Anyway, there's a LOT of free books available. I think curriculum and practice are more important than getting the most modern books. A teacher could really do an awful lot with the free stuff, if he put in the time to think about it.
Also, I found an interesting free app called Study Blue that allows one to create electronic flash cards. I haven't used it yet, but I've downloaded and will look it over later. Interesting idea. My kids make their own flash cards for the languages they study - but the paper cards are bulky when there's a lot of them and can easily be lost.
Irrelevant, but amusing observation:
I notice that there is a free kindle version of "Wealth of Nations," but not of "Das Kapital."