There are millions of ebooks available online, many for free.  Ebooks are a good way to encourage students to read as they can be available anywhere/anytime on any computer or mobile device that the student owns.

Why not put your own school or subject library together?

Take it further...

Services like 'Google Books' are now fantastic.  Students could build up their own collection of books and texts.  They can add comments in the margins and highlight text just like they might in a physical text, however, these can be saved and viewed on any device or shared with others.  Here is an example of a book on Google Books and you can see from the screen shot how easy it would be to start annotating the text (If the image is too small just click on it, then look at the top right):
Of course Google Don't stop there.  They are in the process of 'digitising' every book ever written!  Whilst they are not there yet and you do of course have to pay for many texts as you would in a bookshop this gives you access to an array of books you could only possibly imagine!

Obviously services like Amazon's Kindle are also offering similar opportunities.