want to publish mother's books. Should I have to have registration as publisher?

Well the first question is does your mother want to publish them? You cannot publish relatives' books without their permission. If she has passed, then you need the permission of whoever inherited the copyright from her. That would usually be the children, meaning yourself and your siblings. Everything then works the same as if you were publishing your own book. In the first case though, you'd just be helping your mother publish, not publishing for her. The royalties would go to her and of course in both cases, it's her name on the cover. Just thought I'd state the obvious first.

If you self-publish, the best way is probably Kindle. It's free, easy and readers are more likely to take a chance on cheap self-published ebooks than on overpriced self-published printed books through a website they usually don't know.

Or you can go the traditional route. More on that below, which is the last time I answered a how to publish question.

To be noted, that if these books are non-fiction, there are some differences. For personal accounts, treat it pretty much like a novel. But for academic works, your mother would have to be a recognized expert in the field for her work to interest either publishers or readers. So let's say she wrote a book on a Sociology subject. She'd had to be a professor of Sociology or depending on the angle of the book, a social worker might be enough. But basically, she would have to have credentials. Something to show that her opinion on a subject is more informed than mine.