The Westing Game by Ellen Raskin is one of my all-time favorite books. It has characters of every age, and I consider it a book for all ages. It's also a Newbery Award winner, and there are teaching guides for it out there.
Have them try short stories by classic authors. That way, they aren't reading "books for babies," but something they can really sink their teeth into and build on later. Try the short stories of Jack London, Lewis Carroll, and Edgar Allan Poe. Delicious.
For a magic/fantasy series, get OutCast by Christopher Golden and Thomas E. Sniegoski. It's imaginative and action-packed. It's done especially well with boys who are reluctant readers. The quartet must be read in order:
- The Un-Magician
- Dragon Secrets
- Wurm War
Maniac Magee by Jerry Spinelli is a contemporary read that is memorable, award-winning, and straightforward, as is Nothing but the Truth by Avi. Of the two, I favor Maniac Magee, but note that Nothing but the Truth takes place in high school.
Of the contemporary collections included on my Quick Reads and Short Stories booklist, Twice Told: Original Stories Inspired by Original Art by Scott Hunt really jumps out for analysis, because though the stories are brief, many of them are deep and address gender/cultural identity, orientation, death, and abuse.
The same for 21 Proms edited by David Levithan (stories about prom written by various authors) How They Met, and Other Stories by David Levithan (18 stories all written by David) - short stories with older content - some stories are tame, others more racy.
Also try the Orca Soundings line, which is written specifically for high schoolers who don't want (or can't get through) long books and want contemporary, daring storylines. Think Degrassi: the Next Generation.
For more suggestions, just ask.