Please. Most books and articles don't mention accessibility at all. Accessibility is important but is it really as important as understanding http or other main line topics?
It really should be, people with disabilities can really have trouble navigating websites, the least we can do is make it a relative smooth experience.
I don't doubt that. I even said as much. But one has to understand how to build a web site before they can understand how to build a website for a specific class of individual.
Ideally you should design in and implement accessibility from the start.
Doing the site first then accessibility second usually either means accessibility simply doesn't get done at all (since people forget and/or priorities change before it is done), or it is retrofitted in a really half-assed way.
This is really important stuff (both legally these days in many countries, but also ethically) so giving it second-billing is probably a mistake.