Hothouse: Collections Legends of the Dark Knight tpb - a two-parter that tells how Ivy finds her way back into crime while recovering from her very first "breakdown" and first encounter with Batman. She's still a half normal academic researcher at this stage.
No Mans Land vol.3 - she takes up in Robinson Park, fights Clayface, protects children etc. Awesome artwork.
Ivy & Harley tpb - four self-contained cartoony adventures by Paul Dini and Bruce Timm, along with a short Black&White, it's pretty good if you like that kind of thing. Funny and whacked out and not as bad as you might think really. There's also a painted story written by Judd Winnick but that's kinda lame
Poison Ivy Cast Shadows: if you see this you'll probably wonder what it's about. Ivy convinces everyone that she's 'reformed' in Arkham, she's creating all these miracle plants and then she helps batman solve a case. A good character piece but probably only worth buying if you really love the character. Great art. no appearances from other bat-characters
Black Orchid tpb - Neil Gaiman and Dave Mckean, you can't go wrong. Ivy is only on about four pages but it's one of her best portrayals and the whole story is about plant hybrids including appearances from Swamp Thing and Batman
these are really the best you can get in tpb. There's also Batman & Green Arrow The Poison Tomorrow (1990), and Batman:Poison Ivy, but these are very tough to find. The Poison Ivy tpb is a 1997 book that doesn't tie into Batman & Robin at all even though it was released concurrently. It's about Ivy trying to create a quiet little paradise in the Caribbean populated by her own creatures. Of course it gets firebombed.
You can't o wrong with When In Rome
. The "pop art" villains from Ivy's first appearance actually appear in Resurrection of Ra's Al Ghul
