For me, it's the 'DC Comics Year By Year: A Visual Chronicle
This book literally goes year by year and showcases all the highlights that took place in DC Comics' history. Almost every significant character introduction or change is covered, spotlighting Superman, Batman, Aquaman, Wonder Woman, Flash, Green Lantern, Green Arrow, Justice League, Legion of Super Heroes, Justice Society, Teen Titans, Doom Patrol, Sandman, Jonah Hex, and so many others.
The only omission I saw was the death of the pocket universe Superboy in Legion of Super Heroes, which was a pretty big deal at the time. But every other event I can think of in my years of reading DC Comics is addressed here. It's a great book that brought back a lot of childhood and adult memories for me.
What about yours?