Others recommend MBoC, I personally liked Lodish better. More importantly, cell biology is one of those subjects that can actually be fairly light reading. It doesn't burden itself too much with arbitrarily named cytokines, or the astonishingly complicated biochemical pathways and enzyme names. The processes that govern each aspect of the cell are surprisingly very unique so each chapter is a veritable cornucopia of amazing methods our cells use to solve their problems.

The pretty pictures don't hurt either!