I'll echo peoples' love for Count of Monte Cristo. It's an awesome read that doesn't require too much extra from a modern reader.
For a more niche selection: Lord Dunsany and his material relating to The Gods of Pegana. It's a tough read, but it's interesting to see one of the direct predecessors to Lovecraft's horror writings.
While they were written for children, it was fun to read all of L Frank Baum's works relating to Oz.
I'd also recommend taking a look Baum's novel "The Master Key." Like a lot of turn of the century (1900) sci-fi/speculative fiction, it's interesting what was predicted, how close he came with some things, and how radically off he was with other things.