This is probably The Fionavar Tapestry by Guy Gavriel Kay, consisting of three books:
- The Summer Tree
- The Wandering Fire
- The Darkest Road
From the Wikipedia entry:
Historically the Council of the Mages, headquartered in Brennin, may include up to seven mages, but at the time of the story there are only three: Loren Silvercloak, and his source Matt Sören, a dwarf; Metran, First Mage, and his source Denbarra; Teyrnon, and his source Barak. Each source is bound to the mage he serves by magical rituals and oaths, and provides from his own lifeforce the energy needed to power the mage's magical works. This link can be drawn upon even to the source's death, although this will then render the mage permanently powerless. The Book of Nilsom (a grimoire belonging to a mad mage of the past) includes secret knowledge of an abominable method by which a mage may gain power from more than one source.