One of the most influential books I ever read about web development was Jeffrey Zeldman’s ‘blue beanie book’ Designing with Web Standards (2003). It truly changed the way I thought about and practiced web development.
The other book that influenced me a great deal was Andy Clarke’s Transcending CSS (2006)
The great news is that Andy Clarke has made the his updated version Transcending CSS Revisited: The Fine Art of Web Design available online to read for free. Or you can buy a PDF version for offline reading.