This is a project that's been going on for a good while: Project Gutenberg began in 1971 as an attempt to digitize public domain works - classic literature - for computer-based reading.
Originally the works were mostly done up as plain text (.txt format) as that was the easiest and most accessible. However, with the advent of ebooks / ereader devices that can mimic classic font styles - and with HTML format for ereader apps allowing for graphics/illustrations - more works have been put in more legible, more comfortable reader formats such as PDF, ePub and MOBI.
And as works of public domain - expired copyright, only in the United States however, some nations may not be able to access - these downloads can be had for free. Sure, we're not talking the latest Danielle Steel or Stephen King work here, but we ARE talking about must-reads like Great Expectations, Les Miz, Huck Finn, Machiavelli's The Prince, the Kama... oooh, wait, kids could be reading this...
Anywho. For all ereaders out there, Gutenberg is a great place to expand your e-collection.
Please also donate to Project Gutenberg, it is a non-profit.