The benefit of many things is neither well understood nor objectively measurable.

How do you track the value of friends? The value of meditation?

Heck, how do you track the value (negative or positive) you get from reading this site?

Each of those things affects your mental state in some way, but we have no good way to directly track mental state.

We can track the effect of a good mental state by observing that one is more productive, or marks higher on a happiness scale, but those are second-order effects and there will always be many confounding factors.

We still can't evaluate intelligence nor longevity well enough to track any changes in those regards, and that is likely the area you would need to evaluate for tracking the impact of reading.

If reading makes you feel more content or more knowledgeable about subjects, or if anecdotally things you read seem helpful in conversation or in life, that's probably as good as you'll get.

I personally think that attempting to track and optimize even the most minute details of life is ultimately detrimental, but that's a rant for another time.

Crudely, I guess you could sort of track the "Bayesian surprise" you get by reading a book by measuring changes in skin conductance --essentially tracking how many "frissons" you get while reading maybe? (I really don't know tho...)