There are many sets of accessibility guidelines but we recommend the following guidelines for more technical information:
These top tips will guide you in the creation of accessible EPUB 3 files
Designed to give quick and up to date reference tool, The Quick Start Guide is written in clear, non-technical language and is available in English, French, German, Italian, Korean and Spanish for free in EPUB 3.
Published by O’Reilly Media and available for free download this book provides a definitive guide to accessibility and EPUB 3
The Accessible ebook Guidelines for Self-Publishing Authors provide practical information to help writers ensure their publications are accessible to the broadest range of readers. Published by the WIPO Accessible Books Consortium in conjunction with the International Authors Forum (IAF). these can be downloaded for free.
Produced by EDItEUR in association with WIPO and DAISY this more lengthy set of guidelines provides a ready reference for publishers and other content providers covering non-technical and technical aspects of inclusive publishing. Available in English, French, German, Hindi, Italian, Japanese and Spanish and in a number of formats, the guidelines can be downloaded for free.
All of the above training resources are complemented by this hub of knowledge and best practice for the creation of accessible content
IMS Global Guidelines for Developing Accessible Learning Applications
More technical than other guidance this document from IMS Global is a must read for ebook developers producing educational content