The Library of Borrowed Hearts by Lucy Gilmore

 Library has  2 parallel plots, one of which I liked more than the other. Chloe is a 20 year old who works at a small-town library and is the legal guardian of her three younger sibs. That happened when her single mother decided she wanted a more fun life and walked out, leaving her 3 youngest home alone. Chloe dropped out of college, found a job, and became the new mother of the family. That is the plot I liked. (You can't help loving Chloe.) The other plot starts when Chloe discovers an old book at the library with love notes penciled in the margins.  She's intrigued to know more about the young lovers--and their story alternates with the chapters on Chloe's life. (That was the plot I liked less.) Eventually, Chloe realizes the young man of the romantic couple is her now-elderly curmudgeon of a neighbor, Jasper--who has never married. She's dying to know what happened to Catherine, his true love, but he is not the warm and friendly type. The more Jasper realizes the sacrifices Chloe is making to keep her family together, the more he admires her and soon he becomes a sort of honorary grandfather to the clan and opens up about his lost love. Eventually the 2 plot lines collide with surprising revelations.

Worth reading for the story about Chloe and her goofy younger siblings--Trixie, Noodle, and Theo.