Just some issues I suggest you read to better understand Rachel.

New Mutants #18: She shows up in the 'mainstream' timeline.

Uncanny X-Men # 184: Ray finally meets up with the X-Men when they rescue her from Selene.

Uncanny X-Men #199: Rachel accepts the Phoenix Force and becomes Phoenix II.

Uncanny X-Men #207: On a bit of a power/guilt trip Ray tries to kill Selene. Wolverine stops her and she dissappears.

Excalibur: The Sword Is Drawn: Comes back from the Mojoverse being chased down, helps to found Excalibur.

Fantastic Four Annual #23, New Mutants Annual #6, X-Factor Annual #5, X-Men Annual #14: Days of Future Present storyline.

Excalibur #11-24: Cross-Time Capers, a really crazy journey across time.

Excalibur #50-52: Battles Necrom and dies, but is reborn. Her background is revealed by the Phoenix Force.

Excalibur #61: The Phoenix Force using Ray's body takes on Galactus and knocks him for a loop. Wonderful splash pages.

Excalibur #64: Rachel regains conciousness and has a heart-to-heart talk with the Phoenix Force. Now wears the Dark Phoenix outfit.

Excalibur #66-67: Days of Futures Yet to Come storyline. Is known to Bishop as the 'Summers Rebellion'.

Excalibur #71: Reconciles with Jean and gets to know her brother Cable better after a slugfest.

Cable #9: Rachel and Cable have a more peaceful talk and acknowledge the bond between them.

X-Men #30: Her parents finally tie the knot.

Excalibur #75: Switches places with Captain Britain and is thrown into the future.

Adventures of Cyclops and Phoenix #1-4: Old and dying two thousand years in the future, she takes her parents and gives them a twelve year honeymoon.

Cable #65 and #71: Rachel in her Mother Askani persona visits her brother and shows him some important things.

X-Men: True Friends #1-3: A limited series featuring Kitty and Ray during WWII Europe.

X-Men: Phoenix #1-3: Rachel's own limited series.

Cable #82: Rachel makes a desperate telepathic cry for Cable from a mysterious place. And she's wearing the Dark Phoenix costume.

Cable #85-86: The upcoming "Search for Rachel Summers" storyline that will herald the return of Rachel (not as Mother Askani).