I nominate "
The origin of species" by Darwin.
I read a good review of it in the paper two days ago, which stressed its good language and general accessibility, so I thought it could be interesting to read.
It's also a book that has made a great impact, and knowing what it is about first hand, can only be a good thing. Finally, it's Darwin's 200 years birthday anniversary this year, so I thought it could be extra fitting to read it.