> the author has set himself to read 20 books per year. Suppose this is double of his standard.

That would be an obvious misstep. Both in terms of objective setting and in measurable key results. This blog post misses part of the value of OKRs: they teach us to set high level objectives and repeatedly decompose them. Here, objectives are instead just categories that provide no focus. It's like that antipattern where managers ask their directs for projects then try to combine them into OKRs. What, pray-tell, does launching an open source project have to do with boosting knowledge? And like, why 20 books instead of 10? And why specify quantity rather than quality or subject? Does reading a book mean you've boosted knowledge if you barely remember anything you read?

A better OKR setup would likely be something like (a bit modified to suit my own circumstances):

   Objective: Boost Knowledge
   Key result: learn more about statistics

etc

Which is decomposed into:

    Objective: Learn more about statistics
    Key Result: Work through the Freedman, Pisani and Purves Statistics textbook
    Key Result: Create a hundred Anki cards on statistics
    Key Result: Make the front page of HN with a stats blog post

And even those can be subdivided

    Objective: Make the front page of HN with a stats blog post
    Key Result: Generate ten blog post ideas
    Key Result: Research and analyze five blog post ideas
    Key Result: Publish and promote all five blog post ideas

But you're absolutely right that meaning can be lost. Moving passion projects into a todo list misses the 'passion' part entirely, and is best reserved for the drudge parts of life necessary to enable the sublime. By all means, set OKRs for retirement, taxes, and career development.