Just five is hard. I'll probably change my mind several times as I think of some others. But here's five I like to keep on my book reader:
1. A Song of Ice and Fire (the whole series), by George R.R. Martin; at Sony Connect, Fictionwise, etc.
2. Dune (just the original one, not the series), by Frank Herbert; at Connect, Fictionwise, etc.
3. The Drawing of the Three (again, not the whole series), by Steven King; at Connect, Fictionwise, etc.
4. Tao Te Ching, by Lao Tse, (also called "Sayings of Lao Tse, etc.); at Mobileread, Connect, Fictionwise, etc.
5. The Prince, by Nicolo Machiavelli; at Mobileread, Fictionwise, Connect, etc.
I wanted to include the Gospels of the New Testament but I've never found a good edition in eBook form. Connect sells an edition translated for teens but I don't feel it does justice to the material. King James was sort of an English Caligula and I don't trust that translation. The New English Bible is good but not available in eBook. Well, better quit. I'm starting to dither.