Top tip - don't waste money on expensive electronic books like the Kindle or Sony eReader. Just get 100 Classic Book Collection for the DS and get a library that plays Puzzle Quest!
Ok, in much the same way that the lack of an official digital release of the Beatles' back catalogue hasn't kept the White Album from many iPods there are already several ways of reading ebooks on the Nintendo DS - but they are all homebrew efforts and not strictly on the level.
100 Classic comes with (hey!) 100 books from the public domain like Pride and Prejudice, Treasure Island and A Tale Of Two Cities with the ability to download more titles via Wi-Fi. Titles are searchable and bookmark-able and a built in book guide can recommend titles for your perusal.
Nintendo's DS e-reader priced and dated [Electricpig]