I mostly read speculative fiction. My favourite authors are Iain Banks, Philip K. Dick, J.R.R. Tolkien, Terry Pratchett, Adam Roberts, Urusula K. LeGuin, Peter F. Hamilton, Dorothy L. Sayers, Umberto Eco, Anne Rice, Alastair Reynolds, Kim Stanley Robinson, William Gibson, Harry Harrison (His old material especially), the sci-fi big three (Clarke, Asimov, Heinlein), Philip Jose Farmer...etc.
Out of all those, if I was to pick five books you MUST read before death:
Ursula K LeGuin - The Left Hand of Darkness
Iain Banks - The Bridge
Harry Harrison - Bill, The Galactic Hero
J.R.R. Tolkien - The Lord of The Rings (The films are NOT a substitute. Force yourself through the dull bits)
Kim Stanley Robinson - The Years of Rice and Salt
Get crackin'