Common characteristics (legs, mammary glands) can take disparate evolutionary paths for all sorts of reasons. The fact that some men find breasts attractive doesn't mean that sex is the reason for obvious breasts.
No, but there is usually some evolutionary advantage to a new characteristic for it to persist once it has appeared largely by chance, I think that is accepted widely. Obviois external breasts can be cumbersome when running, and could be vulnerable when you are attacked, so there is very likely something in their favour, or they would have tended to fade out over generations.
Women may well have spent more time lactating in the past, but many animals spend a high proportion of their lives feeding young too, so if that was the reason we have them, many animals would also tend to have breasts from puberty.
The most common theory on why they persisted, is that they were attractive to men - in fact I cannot find any other theories on why they evolved that way. Would be interested if there are other theories, but even then, this is considered the most likely reason currently.
As I said, I don't think the wording in the book is particularly a good idea for small boys, but that doesn't make it true that breasts are there, sticking out on women's chests, just to feed babies.