It might seem silly to discuss a title as self-explanatory as this one, but we like to leave no stone unturned. Giovanni uses her title as a dedication, telling us that she means to praise Mrs. Long in the body of the poem. In that way, our poet creates a kind of ode, which has some very ancient literary roots. Giovanni's poem doesn't follow the formal structure of an ode (as we discuss in "Form and Meter," she's writing in free verse to create a rhythm and structure of her own). All the same, she's clearly sending a shout-out to her former librarian.
There's also a parenthetical add-on to the title: "(You never know when a troubled little girl needs a good book)." We don't really get a sense of the speaker's turmoil in this poem (remember how happy she is skipping home and getting neck kisses and reading her books on the porch?). But if we look at the whole picture, we can see the potential for trouble. If Mrs. Long hadn't been there for her, if she hadn't braved the unpleasantness of discrimination uptown, then where would Giovanni be today? What would have happened if grandma hadn't made her feel safe and loved, if Mrs. Long hadn't encouraged her to look beyond her own world? The title of this poem, then, is meant to acknowledge with gratitude all the hands that can lift a young girl to a new level.