I want to learn about legal minimum requirements for accessibility

This is a great place to start. We have summarised accessibility legislation here: 02 Accessibility Legislation and have separated the information out by country. We have pulled out the key aspects we think you will be interested in: describing and linking to relevant legislative acts, naming the accessibility standards they specify, interpreting how it applies to small publishers, summarising how compliance is checked, detailing exceptions and some notes about national copyright considerations. It's important to stress that we are not legally trained and we therefore cannot give definitive legal advice.

Once you have wrapped your head around legal requirements, you might like to move on to 04 Planning for Accessibility or jump straight into working on achieving the legal requirements using The OBF Open Accessibility Strategic Planning Model '10 Steps to Creating Custom Accessibility Roadmaps, which is also available as a downloadable document on this page: 06 The Open Book Futures Accessibility Tools