4. Mordant's Need series by Stephen Donaldson
Of course, consisting of only two books this isn't really much of a 'series'. But I wanted to include it anyway because of its fantastic characterisation and its marvellous fantasy thriller-like story. I also love the unique "imagery" magic system used within the book. You see, on the world of Mordant magic is all done with mirrors - literally. Mages have to construct large, warped mirrors which then act as magic doorways allowing them to conjure items, elements or monsters from other realms or transport objects or people to these other places. It's a fascinatingly limiting method of conjuration put to great use in summoning monsters, fiery lava and unusual champions - such as Teresa, the passive protagonist of the book.
I remember really getting caught up in the story - a lighter, easier-to-read offering than Donaldson's other books (I found his Thomas Covenant books a bit dark and difficult to get into) - wishing that Teresa would stop falling for the manipulative Master Eremis' charms and become the hero she was meant to be. It was a thoroughly gripping read and one I still fondly remember.