Depends on what kind of books you like man.
Ive read 'In the Name of the Rose' when I was in senior high school. And mind you , im a HEAVY reader.
That book bored me to tears. I finished it just because my pride made me do it.
I always reccomend the DUNE books written by FRANK HERBERT (not the prequels written by his son after Frank's death.. his son cant write worth snot).
The DUNE books seem to many as sci-fi stuff, but its really not sci-fi in the way you think of asimov (I, Robot, Foundation books) or Heinlein (Starship Troopers, The Left Hand of Darkness).. Herbert takes religion, sociology, psychology, politics and human nature and uses them to move an incredibly complex story along.. the only 'sci-fi' elements are that its based 10k+ years in the future and there's space travel..thats it.
If its sci-fi you like, you can look up the REALITY DYSFUNCTION novels (I forget the authors name).
Fantasy books you can look up DEATH GATE by Margaret Weis & Tracy Hickman... its not the same thing as their famous DRAGONLANCE books (which are corny as heck in my view since they just stole the stereotypes of the fantasy races and made the lame story around it, no matter how well written it was it was still LAME). Deaths Gate is an excellent set of books imo.
You could also try reading books from other cultures or 'based' on other cultures.
SHOGUN (James Clavell) is an excellent book. Written by a foreigner, its about a British sailor who gets stranded in Japan's feudal era. Fiction
Tale of Genji (Japan) is AMAZING in detail of life in feudal japan, as written by the Lady Genji, a member of the imperial court. Non-Fiction.
TAIKO (Japan) narrates the life of Hideyoshi Toyotomi; the man who united all of Japan under one rule. Historical fiction (aka, facts and events in the book are historically accurate, the characters in book are based on what was known of the personalities of the historical figures)
100 Years of Solitude (Colombia) by Gabriel Garcia Marquez is the story of a family before, during, and after the time known as 'La Violencia' (The Violence) .. Colombia's civil war. Follows the lives of several generations of the family in those times. Very good book. Fiction
War and Peace .. this one i wont even try to summarize. Its an effing big book.
MAUS (graphic novel by ART SPIEGELMAN) one of the hardest 'books' ive read.. its about the Holocaust. I think this graphic novel should be required 'reading' in all high schools.