frankenstein/dracula/dr. jekyll and mr. hyde
a trio of books we were required to read in my sophomore english class... frankenstein was by far the best, in my opinion. the original story is nothing like it would be later portrayed, and you're right, it does present a lot of moral dilemmas and psychological analysis -- as well as some very interesting uses of description and chronology. i think even more than the novel itself, i find the story behind how it came to be much more fascinating. the story goes that mary and percy bysshe shelley, lord byron and other social notables were caught overnight in a storm, byron challenged each member of the group to write a story of the "supernatural"... and frankenstein was mary's contribution. of course, that's just the quick version, the expanded account of how this all came to be is quite interesting, so you may want to read up on it as well.
(note: here is a very interesting online exhibit about the book and the author -- check it out.)