The Phoenix has returned, weak and disorientated looking for it's host Jean Grey. It's up to the X-Men to hunt her down and with the help of the Shi'ar destroy her once and for all in a final confrontation.
The art works really well for this story and is great to look at, however before you start praising Greg Land be aware that he is a serial tracer. There's a reason the images look almost like a photo (just Google his name to get the full story on that).
I'm not a long time follower of the X-Men, however Greg Pak does a fantastic job on the writing. He seems to have captured all the characters, the Phoenix itself really stood out. The squeal to Endsong (titled Phoenix - Warsong) is also written by Pak.
Although being a reader of the New X-Men comics would be helpful in understanding this graphic novel you could just find the back story on the Internet. If you do want the build up to Endsong I would recommend buying New X-Men Volumes 4, 5 and 6.