Faithful Place by Tana French. One great thing about Tana French's interlocking mysteries set in Dublin is that you can start with any one. An even better thing is how well they're written. This story of a detective drawn back into a mystery from his teen years will take you back and keep you up.
REAMDE by Neal Stephenson. A computer virus, hackers, gamers, mobsters, religious fundamentalists. A fact paced global adventure that weighs in at several pounds.
Haroun and the Sea of Stories by Salman Rushdie. A fable about the power of story. Funny and sad by turns. Perfect for reading aloud.
Thud! by Terry Pratchett. This more recent addition to the Discworld canon is a perfect place to plunge in. Old tensions between dwarves and trolls are heating up and Commander Sam Vimes of the City Watch is caught in the middle. Add in wizards, vampires, and werewolves (and that's just the watchmen) and you've got a caper worthy of this comic genius.
Why we Broke Up by Daniel Handler, Illustrated by Maira Kalman. A post-mortem of a high school relationship told through the box of mementos left behind. Gorgeous paintings and a story that highlights the difficulty of connecting with someone while caught in the social landscape of your peers.