I'm going to set aside the question whether books are worth the money when there's free advice online because the value of a dollar is a pretty individual question. Plus there's always the library.
What I reject is your premis that the principles of training are "known." Maybe at a very rough level that's true, but if that's what you need, then you're a novice who might struggle to identify reliable info online. There's a good chance you end up doing HIIT or something.
Assuming you have baseline knowledge, then running books are food for thought. You get a detailed, comprehensive, and sustained look at one person's approach. You can reassess what you already know or think you know you know in light of that. You can also identify points of disagreement and agreement between various coaches. Where there's unanimity, you should probably follow their advice. Where there's disagreement, you can experiment.
Personally, the books that influenced me the most are Daniels Second Edition, Canova's marathon book, and Olbrecht's The Science of Winning (a swimming book). I initially bought into Daniels completely. I now think the basic premise is wrong, but there are still pieces of it that I find useful. Canova's book gives you bits and pieces without any real plans, but I still buy into his basic approach to periodization and emphasis on extending specific endurance. And for any distance shorter than the marathon, Olbrecht is indispensable for coming up with the proper balance of aerobic and anaerobic training that's individualized for each athlete.