It depends on what kind of geek you are. "The Ants" is perhaps my favourite non-fiction book of all time - at the time it was written it was a fair account of the total sum of human knowledge of ants. It's beautifully written and incredibly complete, and although it's now a bit dated, it's still the best reference to ants if you're interested in them. For a broader summary of Wilson's intellectual project I recommend "Consilience" - a wonderful synthesis of almost all his ethical and scientific interests.
His life, the studies he undertook quietly over protracted periods of time . . . there is no "one book" . . . his mind was a burrowing mind . . .
If you appreciate logic, well-reasoned arguments and objective observations --the scientific method personified -- you will savor anything related to his endeavors.