As YOASOBI grew in popularity after its runaway 2019 hit, “Yoru Ni Kakeru,” so did the sources of inspiration behind the synth-pop duo’s singles, collected here in THE BOOK 2. While their songs were based initially on online short stories, producer Ayase and vocalist Lilas Ikutas began to contribute works as tie-ins for other media, including anime blockbusters and TV programs for children. The scale of their music matches the rise in their profile, best showcased on “Monster,” the duo’s first theme song for an anime series. The snarling beast of a track buzzes awake with a mean electro riff that towers over the rest of their delicate synth-pop. But no matter how much Ayase builds the density of his busy production, YOASOBI’s songs move just as swiftly and Ikuta sings the zigzagging melodies just as brightly. On “Romance,” she nails every hairpin turn, breezing through the dramatic string hits and cascading pianos that send the music skyward. The concepts behind the songs of THE BOOK 2 turn grander, the music becoming more sprawling, yet the duo continues to nurture a one-on-one closeness. As Ikuta ruminates on the importance of human connection, her lyrics often use the second person, as if she’s directly opening up to her listeners about the relationship they share. The more stately slow jams carve out a cozy space, optimal for closer listening to her heartfelt words. While the singer reflects on the good gained from a soured romance on the pensive “Moshimo inochiga egaketara,” she celebrates the friendships that uplift her during a time of emotional need on the sunny piano-pop of “Mou sukoshi dake” and “Comet.” While YOASOBI’s growing popularity elevates the duo’s music to new heights, their lyrics on THE BOOK 2 preserve the intimacy inherent in their songs since the duo’s bedroom-pop days.