Yeast by White and Zainasheff. granted White (of White labs) has avested interest in promoting how important yeast is to brewing, but it is a facinating book. It is a bit rough in places since it was written for craft and homebrewer alike, but very informative. There was at the time some question I had which the book didn't answer, and makes me think it wasn't important.
I also came away with the thought that there were several phd level thesis in chem or bio that could be persued based off things that were in the book.