I believe you have it! You are speaking of "observed effects" - or "observable actions" of an unobserved phenomena - in this case a falling book providing evidence for the Theory of Gravitation. This is good.Observation that replicate itself and form a similiar pattern create credibility. Thus a observed action can determine if a observation has credibililty. For example (simplified) push a book off the table it falls down and the similiar action prevail you can make a valid conclusion. If the book floats into the air then the first conclusion is nullified. But if in fact book fall off the table onto the floor you have observed gravity in action. Is that simple enough?
Now - let's extend this to the Theory of Human Evolution. The "observed effect" or "actions" of human evolution are the 25 different species of fossil hominids. Their pattern shows a change from ape-like to human-like over time. If these fossils fell into a different pattern - like your book floating in air - then human evolution could be nullified. But they don't - they fall into the exact pattern of change in which you would observe human evolution in action - just like your example of the book falling to show gravity in action. And each new fossil found falls into this pattern.
So you were right! The repeated observations of the actions/effects of human evolution - those homininid fossils - continue to fall into the same pattern predicted by evolutionary theory, thereby creating credibility.