Not easy! Especially when there's so many books not available as ebooks.
I guess essential means different things to people, but for me it means books that are just not my current favourites, but books that may have a bit more to offer, and is, if possible, also of some significance.
On the Origin of Species, 6th edition
A Room of One's Own
Fiction, ancient literature
More info on Wikipedia
A Passage to India
E. M. Forster
I actually wanted "Maurice" on the list, but it's not available, and so another of Forster's novels would have to do.
His Dark Materials Trilogy
Fiction, young adult