Blue Moon: The Immortals

Blue Moon was written by Alyson Noel.  It's is book 2 in The Immortals Series.

From alysonnoel.com:

Just as Ever is learning everything she can about her new abilities as an immortal, initiated into the dark, seductive world by her beloved Damen, something terrible is happening to him. As Ever's powers are increasing, Damen's are fading—stricken by a mysterious illness that threatens his memory, his identity, his life.

Desperate to save him, Ever travels to the mystical dimension of Summerland, uncovering not only the secrets of Damen's past—the brutal, tortured history he hoped to keep hidden—but also an ancient text revealing the workings of time. With the approaching blue moon heralding her only window for travel, Ever is forced to decide between turning back the clock and saving her family from the accident that claimed them—or staying in the present and saving Damen, who grows weaker each day...

Loved Summerland.  I loved the way they pulled in the past... and how Summerland gave Ever (and the readers) a way to see the past without them really being just "flashbacks".