> but the book's font is smaller.

If the book's font is smaller, but the lines are the same width, that means more words per line in the book, yes?

And each line is taking a larger fraction of the visual field, since the book is closer to my eyes.

Basically, if we assume books are somewhat optimized for ease of reading (and to a large extent they are), websites should be aiming for the same angular sizes as books. The reading distance is larger, so the physical sizes should be larger: larger fonts and wider lines...