Man, I had to read that one a few times. It means that Tarou bought a book in addition to some other activity, which might have been buying something else, or might have been some other action other than buying. Because of the implication of "additionality" (just made up that word), the most likely context is that he'd bought something else. But there could be other actions, though to bother using "mo" there'd need to be some kind of implication of "additionality". Like say if it was a library where you could either borrow or buy books, you might be able to say "The children all borrowed comics. Tarou also bought a book".I think you're right about the usage not being identical across the two languages, but in this case I think what matters is getting to the same functionality by some means or other. I've had a couple too many beers to be confident about that, though.