I haven't read Nicholas Carr, but I've seen summaries of some of the studies used to claim that book reading results in more comprehension than hypertext reading. All the ones I saw are bogus. They all use, for the hypertext reading, a linear extract from a book, broken up into sections separated by links. Sometimes the links are placed in somewhat arbitrary places. Of course a linear text can be read more easily linearly.

I believe hypertext reading is deeper, and that this is obvious, almost true by definition. Non-hypertext reading is exactly 1 layer deep. Hypertext lets the reader go deeper. Literally. You can zoom in on any topic.

A more fair test would be to give students a topic to study, with the same material, but some given books, and some given the book material organized and indexed in a competent way as hypertext.

Wide and deep reading, such that you make the information presented yours, gives you more background knowledge that helps you find your own connections.

Hypertext reading lets you find your own connections, and lets you find background knowledge that would otherwise simply be edited out of a book.