There are three versions of the Elektra: Assassin TPB. The original soft cover and the original hardcover (very limited), and the most recent new printing (soft cover).
Then there is the Elektra Saga TPB, which reprints Elektra Saga # 1-4, which are altered reprints of Elektra's appearances in Miller/Janson's DD issues, so that the story is seen from Elektra's POV.
Do you regard the Elektra Lives Again graphic novel as a TPB? This exists in at least three versions: two hardcovers (one from Epic, one from Graphitti Designs), and one recent softcover standard format printing.
I'm not sure if the Elektra: Root of Evil mini-series exists as a TPB (and I don't know if any of Milligan's issues were collected, either), but she appears in the DD: Fall From Grace TPB, which (as the writer himself recently informed us) has 15 extra pages of story and art (though it doesn't say so anywhere on the TP

.
Then there is the recently solicited hardcover printing that collects Elektra: Assassin and the Daredevil: Love & War graphic novel. I don't know if this is out yet, but it will be very soon.
I suppose there must also be a collected edition of Bendis' most recent Elektra # 1-6, but I haven't noticed it.
- Tue Sorensen