Hi A.,
Congratulations on your pregnancy. There are books in the Arthur, Berenstain Bears, and Franklin series about new babies joining the family - these are fiction stories for children, not factual books about babies. I don't know about a book that specifically talks about being a brother to a sister, and he doesn't need to be any more gentle with a girl baby than a boy.
As for the breastfeeding, I think this is just a discussion that you need to have with him. Honestly it's not different from bottle feeding - if you were giving the baby a bottle, you couldn't stop in the middle of feeding to get up and do something for your 3 year old - with bottle feeding, you always need two hands, breastfeeding, you get to a point where you have a free hand - way easier when you have an older child. When I had my 2nd, I eventually had both hands free, I dressed Barbie and colored pictures and read to my 4 year old when feeding the baby. I think it's important to discuss with your child what it means to have a baby, and that when you're in the middle of feeding or diapering the baby, you cannot stop. When my son was born, there were times that he'd start to fuss and I would be doing something with my 4 year old, and I'd say, "You have to wait a minute, I am busy with K right now." He didn't understand, of course, but my daughter understood that at that time, she was first, she was the priority and the baby had to wait.
Good luck!!!