Book 3.5 "It's a Nalu Life" is the continuation of Natsu and Lucy's life after book 3, "Light My Fire". (:

Book 3 Lucy and Natsu have been teammates for what feels like forever. And, as much as she hates to admit it, Lucy likes Natsu--a lot. Unfortunately, he seems completely in the dark about her feelings for him. When her plan to confess results in a misunderstanding on Natsu's part, it makes him realize that he also has feelings for Lucy. Though, he thinks he may have discovered them too late.