All of Cory Doctorow's books are DRM-free. Actually, he insisted on it as a contractual rider with his publisher, so he isn't available on any platform that doesn't have a DRM-free option. I wouldn't be surprised if Amazon re-allowing downloads for DRM-free is specifically because Doctorow's publisher is angry at them.
In practice, the biggest store that doesn't have a DRM-free option is Audible... which has a near-monopoly on audiobooks. So Cory Doctorow has to do crowdfunding campaigns for all his audiobooks. Of course, that doesn't stop his books from getting illegally reposted to Audible anyway, and Amazon doesn't care about enforcing rights they can't have. Which led to him actually publishing this gem on Audible: https://www.amazon.com/Why-None-Books-Available-Audible/dp/B...