Sweet Valley University
I absolutely love this series! It's out of print now, so you'll have to buy them used on Amazon or at a local used bookstore. Also Thriftbooks.com is a great resource.
I'll never give spoilers, so don't panic if you read a few reviews ahead of schedule. But be sure to read the books in order : )
Sweet Valley University #1: College Girls
Sweet Valley University #2: Love, Lies, and Jessica Wakefield
Sweet Valley University #3: What Your Parents Don't Know...
Sweet Valley University #4: Anything for Love
Sweet Valley University #5: A Married Woman
Sweet Valley University #6: The Love of Her Life
Sweet Valley University #7: Good-bye to Love
Sweet Valley University #8: Home for Christmas
Sweet Valley University #8.5: Wanted for Murder
Sweet Valley University #9: Sorority Scandal
Sweet Valley University #10: No Means No
Sweet Valley University #11: Take Back the Night
Sweet Valley University #11.5: He's Watching You
Sweet Valley University #12: College Cruise
Sweet Valley University #13: SS Heartbreak
Sweet Valley University #14: Shipboard Wedding
Sweet Valley University #14.5: Kiss of the Vampire
Sweet Valley University #15: Behind Closed Doors
Sweet Valley University #16: The Other Woman
Sweet Valley University #17: Deadly Attraction
Sweet Valley University #17.5: The House of Death
Sweet Valley Uuniversity #18: Billie's Secret
Sweet Valley University #19: Broken Promises, Shattered Dreams
Sweet Valley University #20: Here Comes the Bride
Sweet Valley University #20.5: Running for Her Life
Sweet Valley University #21: For the Love of Ryan
Sweet Valley University #22: Elizabeth's Summer Love
Sweet Valley University #23: Sweet Kiss of Summer
Sweet Valley University #23.5: The Roommate
Sweet Valley University #24: His Secret Past
Sweet Valley University #25: Busted!
Sweet Valley University #26: The Trial of Jessica Wakefield
Sweet Valley University #26.5: What Winston Saw
Sweet Valley University #27: Elizabeth and Todd Forever
Sweet Valley University #28: Elizabeth's Heartbreak
Sweet Valley University #29: One Last Kiss
Sweet Valley University #29.5: Dead Before Dawn
Sweet Valley University #30: Beauty and the Beach
Sweet Valley University #30.5: Killer at Sea
Sweet Valley University #31: The Truth About Ryan
Sweet Valley University #32: The Boys of Summer
Sweet Valley University #33: Out of the Picture
Sweet Valley University #34: Spy Girl
Sweet Valley University #35: Undercover Angels
Sweet Valley University #36: Have You Heard About Elizabeth?
Sweet Valley University #37: Breaking Away
Sweet Valley University #38: Goodbye, Elizabeth
Sweet Valley University #38.5: Channel X
Sweet Valley University #39: Elizabeth Loves New York
Sweet Valley University #40: Private Jessica
Sweet Valley University #40.5: Love and Murder
Sweet Valley University #41: Escape to New York
I'll never give spoilers, so don't panic if you read a few reviews ahead of schedule. But be sure to read the books in order : )
Sweet Valley University #1: College Girls
Sweet Valley University #2: Love, Lies, and Jessica Wakefield
Sweet Valley University #3: What Your Parents Don't Know...
Sweet Valley University #4: Anything for Love
Sweet Valley University #5: A Married Woman
Sweet Valley University #6: The Love of Her Life
Sweet Valley University #7: Good-bye to Love
Sweet Valley University #8: Home for Christmas
Sweet Valley University #8.5: Wanted for Murder
Sweet Valley University #9: Sorority Scandal
Sweet Valley University #10: No Means No
Sweet Valley University #11: Take Back the Night
Sweet Valley University #11.5: He's Watching You
Sweet Valley University #12: College Cruise
Sweet Valley University #13: SS Heartbreak
Sweet Valley University #14: Shipboard Wedding
Sweet Valley University #14.5: Kiss of the Vampire
Sweet Valley University #15: Behind Closed Doors
Sweet Valley University #16: The Other Woman
Sweet Valley University #17: Deadly Attraction
Sweet Valley University #17.5: The House of Death
Sweet Valley Uuniversity #18: Billie's Secret
Sweet Valley University #19: Broken Promises, Shattered Dreams
Sweet Valley University #20: Here Comes the Bride
Sweet Valley University #20.5: Running for Her Life
Sweet Valley University #21: For the Love of Ryan
Sweet Valley University #22: Elizabeth's Summer Love
Sweet Valley University #23: Sweet Kiss of Summer
Sweet Valley University #23.5: The Roommate
Sweet Valley University #24: His Secret Past
Sweet Valley University #25: Busted!
Sweet Valley University #26: The Trial of Jessica Wakefield
Sweet Valley University #26.5: What Winston Saw
Sweet Valley University #27: Elizabeth and Todd Forever
Sweet Valley University #28: Elizabeth's Heartbreak
Sweet Valley University #29: One Last Kiss
Sweet Valley University #29.5: Dead Before Dawn
Sweet Valley University #30: Beauty and the Beach
Sweet Valley University #30.5: Killer at Sea
Sweet Valley University #31: The Truth About Ryan
Sweet Valley University #32: The Boys of Summer
Sweet Valley University #33: Out of the Picture
Sweet Valley University #34: Spy Girl
Sweet Valley University #35: Undercover Angels
Sweet Valley University #36: Have You Heard About Elizabeth?
Sweet Valley University #37: Breaking Away
Sweet Valley University #38: Goodbye, Elizabeth
Sweet Valley University #38.5: Channel X
Sweet Valley University #39: Elizabeth Loves New York
Sweet Valley University #40: Private Jessica
Sweet Valley University #40.5: Love and Murder
Sweet Valley University #41: Escape to New York