Piers Anthony's "Xanth" - large magical land, which basically over lays Florida, Anthony has about 30-something books out now and they keep taking secondary characters and making them main characters. He also explores concepts and ideas that he peruses over in earlier books and ever since a critic said something negative about a pun he put in the book, he now accepts reader puns and comes up with his own to splatter all over his books. (For instance, if someone throws down a dime in front of you, you literally 'stop on a dime'. :))
RA Salvatore's "Drizzit Novels" - about the life and adventures of a dark elf who is a good being in a world where cruelty and evil are the social norm. He breaks the stereotype and has to deal with the consequences.
Diane Duane's "Young Magickers" - about young wizards. The kicker here is that magic is almost a science, with formulae and scientific calculations necessary to complete the spell. She started writing them in the 80's I believe, but has recently started releasing updated versions with more modern slang.
Jim Butcher's "Dresden Files" - a modern day chicago wizard detective. He helps with cases that the regular police and detectives can't handle and you get into his magical world alot. Something I like about it is that he keeps evolving and the bad guy from a few books ago that he could barely handle is now a piece of cake. That's not usually the case. Take "Lord of the Rings" for example. Sauron is always the tuff bad guy and the characters would still have just as much trouble if they faced him again.