I got a bad impression from a superficial look at the book.
The typography is clear and clean enough but I saw three different ways of rendering links (no indication (in the contents section), ALL CAPS (in the body text) and lastly preceded by a diamond character (some other context)).
Taken in conjunction with his choice of directory structure for the pages of the book (completely flat) I get the impression that he is not interested in managing compex information in a sane manner, but merely in making text look good. Disappointing.
In fact, those links look different because they are semantically distinct, and the underlying logic becomes
clear as you read.
The small-caps links are inline references to other sections of the same book; their distinct typesetting distinguishes them from mere text.
Because everything in the table of contents is a link to part of the book, these links are not set in small caps or otherwise distinguished — that's a potentially confusing design choice in theory, but I can't complain because it was obvious enough to me from the context that I should try tapping something.
The diamonds are outbound links to other sites. Given that this is a coherent book and not a collection of random posts, this distinction is handy: while reading Butterick, I am more likely to click a link to more Butterick than to click an outbound link which will cause me to lose my train of thought.
I genuinely hadn't noticed the meaning of the diamond links - I wasn't just making a cheap shot.
In fact they are confusingly diverse: his own mailto: link is a diamond link, as is this URL on the Acknowledgements page: http://practicaltypography.com/contact.html
It's totally acceptable, if a bit Knuthian, to expect a reader to learn a few typographic conventions, but they should be well considered. Don't make the reader hover to find which words are part of a given link.
It is probably worth remembering (as this is HN) that design is partly a personal and subjective thing. I am fairly interested in typography but I wouldn't hold my opinions out as the one right way to do things. One thing I have never thought while studying typography is "I wish this author was more pedantic and conventional, I don't know, maybe like a lawyer?".
