Symbolism, Imagery, Allegory
The journal that Todd inherits from his mother is kind of like his fairy godmother. It pops up when he and Viola most need it (a.k.a. they conveniently remember that they have it), and it gives them helpful clues and moral support. Bibbidi-bobbidi-boo.
Todd's mother wrote it "starting from the day [he was] born, Todd. Till the day she died" (5.17), and Ben and Cillian have been hiding the book away for him (you know, since the Mayor outlawed books). They give it to him in his moment of need, at the beginning of the story when he has to run away. Right away, then, the book represents Todd's isolation—his surrogate family gives him notes from his dead family as he ventures out into the world alone.
The book is supposed to be really helpful to Todd—full of clues and all that—but because Todd can't read, the helpfulness comes a bit too late. For example, when they read Ben's warning—"you must get to Farbranch as fast as you can and when you get there, you must warn them" (21.38)—Farbranch is already long gone. So unfortunately the helpful part is kind of negated by Todd's bad timing.
But what's most important about the book is that it contains Todd's mother's voice. This is matters because it's the only tie to family that Todd has, and it's really meaningful to him. The last words that Viola reads to Todd before they close the book are "yer calling for me, son, and I will answer" (38. 60), and Todd says that this stays "in my Noise forever" (38.61). See? It's a really big deal for him to have this connection to his past. Plus, these might be the nicest thoughts that float around in Todd's usually angst-y Noise.