One book that we checked out recently really captured her imagination. (I'd hear her reenacting the story with her stuffed animals at rest time.) It's called Martha Doesn't Say Sorry and it's about a little otter who doesn't want to say sorry when she does something wrong. Then when she finally does say sorry, she says it so very quietly that no one can hear her. I think the reason that Elisa loved it so much is that Elisa may as well be Martha. Elisa also doesn't say sorry. After we had read it a few times Elisa started playing that part of Martha. She would say "I'm sorry" in her tiniest little voice. And then as the story went on she'd say it a little louder and a little louder. It may be my imagination, but I think that since we had that book, she's gotten better at saying sorry.
I read a couple negative reviews on Amazon that argue that the moral of the story is that you can behave horribly so long as you say sorry... which is of course, not what any parent wants. So here excuse me while I digress for just a minute. In our house one thing that we've found helpful is making Elisa not just say sorry, but also asking her what she can do to make amends for the wrong thing she's done. (Offering a toy, giving a hug, etc.) The point is that a half-hearted sorry flung over the shoulder as she flees from her wailing brother is not enough. She has a part in coming up with the idea of what to do to comfort him. And of course, consequences still apply. Anyway, the point of this post wasn't supposed to be about parenting. I just couldn't help myself after I read those negative reviews.
Another book that had a similar effect was Will Sheila Share? A story about a little girl who is struggling to share. One of her little friends wonders, "CAN Sheila share? ... When she has green beans she can share." Elisa loved it.
Both of these books have simple pictures and short and to-the-point text, so they are great for very young readers.