Maybe. It depends on the context of the book story. 'I love you forever' might be a key phrase that drives the plot, like in a love story where the characters make this promise to each other and then face challenges that test their commitment. Or it could be just a passing sentiment in a larger narrative about different things.
1 In this world we live In this world we all may meet Sometime we'll love and sometime we'll hate But if we live together long enough Someday we'll understand that love is a verb not a noun
--William Shakespeare A Midsummer Night's Dream
2 My love for you has not changed For in this love all things are the same You are the one that has made me whole And with your love I have found happiness
--John Green The Fault in Our Stars
3 I love you not because you are perfect But because you make me who I am You are the light in the darkness And the love of my life
-Liam Neeson The Martial Soul
4 I love you so much I can't imagine my life without you You are the one who makes my heart beat And the one who makes me feel alive
--Emma Watson The X-Men: Days of Future Past
I Will Love You Forever- Love You To The End of Time by Tears in Heaven