It’s important to note that PDF/UA is neither a spec to measure PDF content, like the Web Content Accessibility Guidelines (WCAG), nor an everyday authoring guide. It focuses on giving developers of PDF authoring tools and viewers, as well as vendors of assistive technologies that support PDF, critical information on how to build and present PDF content more accessibly. The goal is to make accessible PDFs easy to author and use, however they are produced.
This post highlights 6 free and good books covering Web Accessibility, accessible content, accessible design and accessible typography . All are online HTML version. Some books has free PDF versions also.