How would I know if its really ok for me to download books from a site?

Ask to see the documents the site uses to acquire the right to the materials they provide or contact the author of a book on the site and see if rights have been granted to the site.

I'm presuming you question the website because you find the offer or the website itself to be somewhat suspicious you question whether the website's company could be licensing the books. If the company is NOT licensing the books, then its activities are likely illegal. If that's the case, you have to be very concerned about this company having your credit card number or other payment information! Even if the company doesn't use it for any ill purpose, the website's security may be lax (if the company isn't following licensing requirements, why would it follow credit card # storage requirements?) and your information could be at risk. That said, the company could be perfectly legitimate! You could always have a look at some of the book titles and then contact the publisher to ask. Pick a small publisher, as a small publisher is more likely to respond. Don't pay a site you can't trust.

I would suggest you get the name of a publisher for the next book you want to buy from this site. Then contact the publisher by email and ask if the site is authorized to sell the book, giving the publisher the name of the book and the author, an the web site name. If the publisher says the site is authorized, that will be your confirmation. If the publisher says the site is not authorized, then you should not buy any more books from this web site.

Most legitimate websites of this sort will have a Digital Millennium Copyright Agent. LawQA does not allow us to post links, so Google search on "Directory of Service Provider Agents for Notification of Claims of Infringement." If the site is on this list, it is most likely legitimate.