FAQ is about how Recaptcha works.

About Us lists a bunch of people, a bunch of companies and open source software.

PDF article in Science magazine describes their algorithm.

Nowhere does it mention where the results are and how to access them. Until I see results of their work, or at least any references to those, I am not convinced participating in Recaptcha does any good to the world.

Where can I find books that were digitized with the help of Recaptcha?

I tend to think using Recaptcha just earns somebody money, it is not really doing any particular good for the world.