Harry Potter... I didn't even touch them until I was in my 20's because I HATE the movies, but I was bored one day and picked the first one up. The rest is history! The world J.K. has created is just so detailed and wonderous!
The Southern Vampire Mysteries series (Sookie Stackhouse/Ture Blood)- Love the lore, the setting, the character development... Disappointed in the last few books though.
I feel the same way about the Stephanie Plum series. Great books, I actually laughed out loud more than once(a first for me when reading), but they get dumber and dumber as the series goes on.