The shopping is less for the books, and more for any neat magical items we might find. There are probably tons of neat things that might be useful in some situation, and given both our limitless storage space, and our almost limitless money, we can get those things. I'm particularly interested in what's down knock-turn alley if we can go shopping down there, especially particularly the Vanishing Cabinet if it's on sale, since that links to the Room of Requirement which also has lots of neat stuff.
We have no in character reason to go down a shady alley we know nothing about, and we don't need the Vanishing Cabinet to get into the Room of Requirement - we just really need a place to train some particular skill, and the room should provide if we need it enough. Since the Gamer always needs to grind, I'm thinking it might be enough.
Also, that Vanishing Cabinet is not inside the Room of Requirement at this time. It's also broken. Draco moved it in and repaired it during sixth year. As such, it's rather useless to us at this time.
We can't shop for muggle and magical stuff with the same action, and unless we're going to do something that generates some muggle money we'd have to take another action doing odd jobs around the neighborhood which makes it into two actions. I think we've got enough muggle books for now. We can always go on another shopping trip during Christmas break (with Moody living with us, we have a reason to go back) after we'll likely have gotten some muggle money from dungeon runs.