I find it difficult to believe the captcha served over the last year or so were actually scanned from books: they were so completely illegible and nonsensical. I usually had to click refresh about half a dozen times before I could even find a sample that I could read correctly.
Half of the captcha (the illegible nonsense) is the actual test. The other half is usually easy to read; that's the scan from the book. You can actually answer anything for that part and still pass the test, although obviously if you do, you're not helping digitize books.