Originally Posted by
godisawesome
For Huntress, I'd immediately recommend the Paul Levitz trade trade from the New 52, HUNTRESS: CROSSBOW AT THE CROSSROADS; it effectively features both version of Huntress, since it was written for one of them and wound up being retconned into the other. It's also got beautiful art from Marcus To. If you can't find that, try for any collection you can get written by Denny O'Neill. Also, you should consider any BATMAN: NO MAN'S LAND collection if you see the character in there, or BIRDS OF PREY by Gail Simone.
For Spoiler, all the BATGIRL collections by Brian Q. Miller, supplemented by any ROBIN collections you can get by Chuck Dixon and at least some of the other BATGIRL books by a guy named Kelley Puckett, both of the last two whenever you see they feature the character in a substantial manner.
For Green Arrow, LONGBOW HUNTERS by Mike Grell. I'm not an expert on the character, but I've heard good things about that story. If you want to have some fun, try the GREEN ARROW: QUIVER story by Kevin Smith. It's a nice slice of comic insanity.
For Flash, my two biggest recommendations are FLASH: RETURN OF BARRY ALLEN by Mark Waid, and FLASH: BLITZ by Geoff Johns. Pretty much any other collection you can get with either author is also going to be good, it's just that both of these books are really, really good and both have a major impact on the TV show. They do feature a different main character than the show, though, you may want to consider other collections as well. I've heard good things about the first few New 52 collections from a friend.
For Black Canary, if you can find the JUSTICE LEAGUE collections by Dwayne McDuffie featuring the character, they show her a lot of respect, but the bulk of your reading should probably go to BIRDS OF PREY, by both Chuck Dixon and Gail Simone. The former author did a lot to solidify the character's place in a monthly without Green Arrow and introduced the Oracle friendship, but the latter seems to write the best stories for the character in the series.