we first met adam and eliza may in evernight where they were locked in a battle of wills. soulbound is the next chapter of their story, and picks up not long after they were separated by will thorne and st. john evernight in order to fulfill a deal with mab, the fairy queen. eliza may has been living with mab, and she finds she doesn't quite like it. it's as if she's traded one form of captivity for another. when mab's brother mellan comes calling, and eliza is informed that he is her intended groom, she is especially horrified. she knew mellan in her life in the colonies, she worked for him a while, and she is not interested in renewing that particular acquaintance.
when eliza discovers that mab has been holding adam captive and torturing him, she finds that in spite of her feelings for him she can't just leave him behind. adam knows that eliza hates him. which is the ultimate irony because he knows that she is his soulmate, and that without her heart freely given he is destined to live out his life cursed by mab. when he first took eliza captive, he did so without thinking. because she was the perfect match for his soul. he never considered how she would feel about his actions. how she would struggle to forgive him, to accept anything from him. because he didn't follow through on what she had been promised. and because he never explained himself.
this time around adam opens up. he explains that he was once a templar knight, one who had taken a vow of chastity and who refused to succumb to mab's fairy charms. and so she cursed him, but this curse was modified by the angel augustus, who gave him the ability to create life from death in a sense, as well as the ability to see the lights of a person's soul so that he might find his soulmate and cast off mab's curse. when he saw eliza may for the first time he had been cursed for seven hundred and twenty eight years. his time was running out and he was afraid to let her out of his sight.
when he learns of eliza's history, how her ancestors were not only fairies, but were also slaves, he understands how badly he erred in his original actions. and once they share more and more of their past with each other, adam and eliza begin to understand how much they need one another. how being together means everything. how being together means that one of them has to give up everything, even life. and they figure out how much their love is worth fighting for.
as the series has progressed the novels have gotten more and more deeply emotional. the love stories resonating more. and this really works. adam and eliza are meant for each other, once they start actually communicating and figuring this out it's wonderful.