I think its pretty clear the reading books bit was abandoned long ago. I never get non-test words that are in any way a struggle for a competent OCR system. And on the occasion that I do, its impossible for me to read either. If they provided context it would be much more helpful.
As an aside, if you've ever had to solve one of these through TOR and you happen to be running through some eastern european countries... good god those are the most frustrating captchas I've ever seen. Long strings of "mnnmrnrmnm" with contrasting colors and jpeg artifacts... a few attempts at solving those makes me want to kill someone. I feel bad for people trying to do anything on the internet from those countries. I wonder what the rationale is for making captchas nearly impossible to solve in specific regions.
Welcome to the preview of the day where all the networked, statistically self-optimizing IDSes simultaneously turn on us and clean the messy humans out of their technological world.
Google will captcha-block IP addresses (not just Tor) if they get too many queries from them in a short period of time, so that bots can't crawl the search results.