Whether you are new to accessibility or have bags of experience, Accessibility For Everyone by Laura Kalbag is a fantastic book.
It was first published 9 years ago by the excellent A Book Apart, which has since sadly closed down. Fortunately, Accessibility for Everyone is now entirely free for you to read online.
If you are short on time, there is a section on content and design. I'm due a refresher and plan to dip in over the next couple of weeks.
