I have to say, if I had to pick a favourite series of books out of everything I've read, it would be this series.
They're written by Diane Duane and follow Juanita Callahan (Nita) and her wizardly partner Christopher Rodriguez (Kit) as they encounter the Lone Power, the inventor of death. During their encounters they'll meet other wizards in all sorts of places: other places on Earth, other species sharing Earth and many others from different planets all around the universe.
To become a wizard, the Powers That Be must offer you an opportunity to take the Oath when you're young as this offer isn't given to older beings. Once you've taken the Oath you will have to pass an Ordeal which is where you confront the Lone Power. If you win, you're a full-blown wizard, if not...
Wizards are given power to slow the death of the universe, and humans are not the only beings capable of using wizardry.
The series starts with So You Want To Be A Wizard
when Nita and Kit are quite young. I'd recommend that you should be at least high school aged before starting the series as they're young adult novel. The characters are confronting the inventor of death, so things get darker later in the series as it matures, but it doesn't stay like that all the time.
I have to say, one of the books provides a very good lesson when it comes to reading the small print. READ IT 'cause you never know what you may be getting yourself into.
I recently started rereading the series and am currently reading the third book High Wizardry
, which is interesting because Nita's younger sister Dairine has taken the Oath and the book tells of her Ordeal.
There are nine books out now, with a tenth on the way:So You Want To Be A Wizard
A Wizard Abroad
The Wizard's Dilemma
A Wizard Alone
Wizards at War
A Wizard of Mars
Games Wizards Play
They are awesome books. Duane is probably the only author that I would give money to after just seeing her name on the cover of a book. She's wonderful.