I'm a Breastfeeding peer supporter, I can ask one of my colleagues re a story book/suggestions - so I can post their reply a bit later.
What I would suggest is a gradual transition so you can ease your dd onto a routine that suits you without making dd feel like she is being abruptly barred from the breast. My youngest still settles on my chest of an evening (he is 3) and uses boob as a pillow. I understand this must be difficult where you have a young ds to contend with.
What does your dd enjoy other than the boob? Could you go to the library and choose some fave books and set up a compensation prize? Such as special "mum and dd" story time, one to one? Could you involve dd with purchasing a special evening cup, in her favourite theme/character?
Can I ask whether you currently feed both at the same time in the evening?