Okay, this isn't a fair question lumos39!! But I'll answer as best I can ;)

I think it's either the third PJO book (HUGE fan of artimas :3 ) or The House of Hades!!! It was by far one of the BEST romance scenes EVER!!!!! Walking through heck for each other?? True love!! (I'm a big Part shipper X3)

Thank you so much for the question!! I'm in love with answering anything!! And if you're not sure if it's a book-worthy question, you can pm it to me! (Just mention that it's for the book, cause I'll never notice # I'm Blonde xD) feel free, anyone, to answer the question in the comments section!!