So You Want to Be a Wizard by Diane Duane.

This is a book/series where the wizards are all working against entropy, at scales from the small and personal (taking care of possessions, being patient with others) to the enormous (shepherding new sentients, saving a planet). And, as the series go on, the choices aren't easy or costless.

Plus, this is the oath you take to be a wizard:

In Life's name and for Life's sake, I say that I will use the Art for nothing but the service of that Life. I will guard growth and ease pain. I will fight to preserve what grows and lives well in its own way; and I will change no object or creature unless its growth and life, or that of the system of which it is part, are threatened. To these ends, in the practice of my Art, I will put aside fear for courage, and death for life, when it is right to do so -- till Universe's end.