I second both of these statements. Demon Knights is fantastic. Pre-reboot there is great Batgirl material, both the 2000-2006 series and the 2009-2011 one, and also the 2005-2001 Supergirl series from issue 34 onwards.
The first trade is great, the second is good. The third is messed up by a bad crossover and the fourth starts well but turns on a dime into total rubbish halfway through. I wholeheartedly recommend the first trade, the origin story. See if you like that one.