If you are in SoCal fenders might be a winter only thing, or you might consider some clip on fenders for the days that are wet. In NorCal in SF I think fenders could be useful even in summer as the streets can get so wet from the fog its like rain. When it is dry for a while then the ground is wet the worst part seems to be everything gets covered in dirt as the water dries if you are without fenders - your backpack, shoes, the bike, etc.

I use a backpack myself, but plan to get a rack when I get a new bike that can handle one better. I like the backpack IF I don't have to carry to much and its not to hot. When i have to carry extra stuff with me (a book, laptop, etc) the weight of the backpack gets to me and can make my back/shoulders sore while riding. When it is hot, my back sweats to much with the backpack. I imagine these could both be issues for you carrying books and in the heat of socal.