While "Billy Budd" is also quite good (I am a big fan of the opera adaptation by Benjamin Britten), I suggest that readers not be intimidated by the length of "Moby Dick"; most of the chapters are bite-sized, so jump right in! Trust me, it's worth the effort.

There is no bad Vance. There is just good Vance, better Vance, and the best of Vance.

The latter category includes the Lyonesse trilogy, The Dying Earth, Eyes of the Overworld, and The Dragon Masters.

I find The Cyberiad and The Futurological Congress very enjoyable. The Cyberiad is relatively easy reading, while The Futurological Congress is more difficult, but no less enjoyable. Both exhibit a wonderful sense of humor.

Solaris, which has been made into a movie twice, is as intelligent as of all Lem's work, but very slow-moving and difficult (as are the movies!). Read it at your own risk!

Among science-fiction authors, Orson Scott Card is one of the most accessible to readers who are not science-fiction fans, and he has frequently jumped genre boundaries to write horror, religious fiction, and fantasy.

Some good places to start are: Ender's Game and Speaker for the Dead, Lost Boys (horror), Stone Tables (religious fiction), Treason, and Wyrms.

I recommend avoiding: the entire "Ender" series except Ender's Game and Speaker for the Dead, The Worthing Saga, and the "Homecoming" series.

From Wikipedia:

Elaine Pagles' study of the Nag Hammadi manuscripts in the late 1960's was the basis for The Gnostic Gospels (1979), a popular introduction to the Nag Hammadi library. This bestselling book won both the National Book Critics Circle Award and the National Book Award and was chosen by the Modern Library as one of the 100 best books of the twentieth century.

In a different measure of its influence, the conservative Christian Intercollegiate Studies Institute listed it as one of the 50 Worst Books of the Twentieth Century.