The marginal cost of walking down the street is nearly zero. Likewise, the marginal cost of reading a book is minimal. But since we're discussing an industry with very high marginal costs (doctors' time is expensive!), not to mention higher depreciation (libraries have plenty of books that are more than fifty years old; hospital equipment ages a little faster).

You're right. Almost nothing is 'free' by those criteria. Economists like to say that "There ain't no such thing as free lunch."