Symbolism, Imagery, Allegory
Like everything else in Mattie's life, books are complicated. On one hand, they represent all that Mattie can't have in Eagle Bay: adventure, the greater world, and what life is not really like. The Count of Monte Cristo, for instance, is fantastically adventurous, and not really possible (3.abecedarian), and Mattie herself admits that Jane Austen's books have set the bar of romance a tad too high (22.glean).
Mr. Darcy's swoon-worthiness aside, part of the work books do in this book is stand-in as representatives of the vast world beyond the one Mattie knows. In this way, they symbolize how discontented Mattie is with life as she knows it, in addition to how much she yearns to bust loose and explore.
But some of the books—especially the ones which Miss Wilcox/Baxter gives Mattie about rather verboten subjects—represent the possibilities of what literature can do (check out 23.dehiscence and 33.gravid for good places to see this in action). As a burgeoning writer in a time when women are largely discouraged from such pursuits, this is intensely important to Mattie and her concept of self. Words are a means of conveying the truth—the ugly, raw, difficult truth—and getting this truth out into the world matters. For Mattie, words have the power to make the world.
As you probably know, different books mean different things to different readers. We'll leave it up to you to figure out which books mean what to Mattie; while you're at it, we recommend checking in with Weaver's literary preferences too.