Do you ever think about all the great books for programmers, and then form your opinion on the one book that every programmer should read? Maybe a traditionalist will pick Donald Knuth's The Art of Computer Programming. Or maybe Fred Brooks' The Mythical Man-Month, for a more human-oriented take.
If you're really deep into a particular language, then perhaps you really love something like Dennis Ritchie's The C Programming Language or (even for non-Python coders?) How to Think Like a Computer Scientist.
Grigory at Online Hut recently claimed that the best programming book is Charles Petzold's Code: The Hidden Language of Computer Hardware and Software. His reason? It helps you understand how computers work.
So maybe include some reasons why your chosen book is so great, and/or some examples of how it helped you. Maybe the book you choose won't be strictly about programming. Just think about the books you've read and if there's one that stands above the rest as a must-read for any programmer.