Have you ever thought about how difficult it must be for an author to convey the sound and rhythm of a character's speech? What a powerful tool that can be for a novel when conveyed correctly. The other night I got up from reading a book to walk the dogs and realized I was speaking to my neighbor in a speech pattern not my own, but that of one of the characters in the book I was reading. That's how you reel in a reader!
On the other hand, if not done correctly, or overdone, it can also be the way you lose a reader. For every Outlander , there's an "och, me lassie" Scottish romance that lays it on a bit thick. I enjoy getting into the flow of the Black Dagger Brotherhood series  lingo, but many find the phrasing (and the names) to be distracting. I won't even go into what fiction has done to the accents of New Orleans.
I think the books below have done a good job in evoking the color and importance of speech. Do you have any books to share that immersed you in the language of a place or made you think "Nobody talks like that!"?