Jim Butcher - two series, "The Dresden Files" and "Codex Alera"
Dresden Files is about a wizard who is something of a detective in Chicago. It's got some fun characters, and is a gripping series. There are 12 books now...? This magic is learned, magic words, constructed spells sorts of things. There are Vampires and werewolves and Faeries.
Codex Alera is a Roman-esque historical-ish epic peoples' war/peace saga, with some magic. The magic is elemental and personal and instinctive. There are monsters and drastically different races that agree/disagree/find ways to work together against common enemies. There is some technology and teaching/learning, and a bit of romance.
Kate Griffin - the "Matthew Swift" series: Urban magic in London. Really interesting. Magic based on urban life and ability to "see things differently". Lots of bleeding and death, tho our protagonist is a really decent guy.
Terry Pratchett. All of it. The Discworld series is a start... There are Wizards, Werewolves, Vampires, Trolls, Dwarfs, Witches, Death (and his Granddaughter Susan), Coppers, Fairies, Imps, Dragons, etc etc etc etc. Young Adult friendly, tho they likely won't get all the jokes. Married friends of mine cannot read this solo... they're all so funny that you'll get tired of asking "what's so funny?" all the time, so they just cut to the chase and read them aloud. It's a parody of the modern world, and of Fantasy novels, and it's just plain fun.
"Good Omens", by Pratchett and Neil Gaiman (previously mentioned, also fabulous) is one of my FAVORITE BOOKS EVER.
"Lamb", by Christopher Moore, also one of my FAVORITE BOOKS EVER. Fantastic. His other stuff is fun. Crass, a bit, but fun. Not necessarily serieses, but the characters tend to pop up frequently between books. I just re-read "Coyote Blue" and Loved it even more than the first time.
Gail Carriger's "Parasol Protectorate" series is a good time, also. Vampires, werewolves, steampunky.