What books do I get before reading doomsday clock?
The most essential book you need to read first is Watchmen. In many ways it’s more of a sequel of that.
Outside of that, you might want to read the DC Universe: Rebirth one-shot, as well as The Button crossover in Batman and The Flash. Those are the only books that really come to mind.
What JK said.
For a good deep dive, you could check out the “Before Watchman” series, unfortunately I don’t think they are available on the service.
DD Clock actually had me doing a lot of research on characters as they made appearances. I really like this series!