Per Comichron (not the entire story, but the piece of the story we get):
Issue ranking / dollar rank ordered
Harleen #1 11 / 4 84,785 + 5,202
Harleen #2 23 / 7 65,720
Harleen #3 14 / 7 59,501
Add digital sales and other non-Diamond channels, and it's a really solid seller. Given this, and reaction from critics and fans, DC ought to be doing their damned best to negotiate with Sejic to get the next two volumes out ASAP (I doubt volume 2 will arrive that much quicker, but they can probably shorten the wait for volume 3). I hope Sejic gets decent royalties, too.