personally I'd sum it up this way: it is usually enough to abstract two parts of your code: the repetitive stuff and the stuff that can be separated from external dependencies like db or network. That should be enough to ensure readability and that you can test it properly and not have to deal with rewriting half your codebase when you decide to change an external dependency.

Can you give examples? I genuinely can't think of how the principles applied with proper restraint not to overdo it make code hard to maintain. But I've only watched his talk a few years ago - not the book.