I make a habit of finishing everything I read, to exercise my ability to maintain focus. But that only works by being extremely selective of the material I begin, particularly books.

If a book is skimmable, why bother? Presuming the author has something genuinely interesting and substantive to say, there are often better alternatives. For example, books written by scholars are often preceded by long-form journal articles which more concisely present the material. And if you were mistaken in your belief they had something interesting to say, then you've only wasted 20 pages of your time, not 400; yet you can still say you gave them the full measure of your attention and a fair chance to persuade you.