This is from the Identity Crisis miniseries, by Brad Meltzer and Rags Morales.
Boomerang is first introduced in #2.
In #3...
In #4...
Later...
In #5...
Later, Jack Drake receives a package from someone who knows he's Robin's father, with a gun and a note telling him to protect himself. Worried, he calls Oracle.
In #6, everyone assumes that Boomerang was the man who killed Sue Dibny and tried killing Jean Loring.
In #7, it's revealed that Jean Loring, who accidentally killed Sue Dibny and made it look like a murder, is behind the whole thing.
Also, does anyone know when Digger got overweight?