While this is certainly good news, I remain cautious it will make much of a difference.
For many of us, we would much rather stay in the beautiful Apple eco-system whenever possible. But for Books, the Apple store is a bit of wasteland, where they don't have all of the books Amazon has and far more important is they don't have the reviews and discussions Amazon does. If I lookup a book on Amazon I can see hundreds or thousands of reviews and quickly know the positives and minuses about the book. With Apple the same book may only have 5 reviews or none, so not a great enough sample to know much of anything about the book. When Apple missed out on buying GoodReads it further sealed their fate.
Apple can continue to improve the UI, keep costs competitive, add new fun features, but people use Amazon for the reviews/community, and Apple can't build that without first having more reviews and a community. Buying something like a Good Reads gives a company a major head start, but currently there are no other GoodReads big enough and Amazon certainly won't sell it.
Books will remain popular for the millions who always use the defaults app, but will lag behind Amazon, unless they truly push getting reviews or other must have features, color e-ink screens, or Amazon maybe sells the business to focus elsewhere.