The letter label corresponds with the row
Viewing post in The Grand Book Archive: Tidy Up comments
I know. But it doesn't correspond to anything about the title of the books.
In real life, books are sorted alphabetically by title, author, or series, within the categories of whatever system the library uses (frequently the Dewey Decimal System or a derivation thereof).
In this game, the title of the books is irrelevant to the placement on the shelves, and a random additional letter is used to place them in pseudo-alphabetical order.