I would not say that this site is legitimate. If you go to their DMCA/Copyright page (link at bottom left), you will see this statement:
Our site is "search engine" - automated information location tool.
We do not host any book files on our servers. Site is search index archive with a convenient browsing capabilities. Site offers "Books Manager". Books Manager is a software for searching files anywhere on the web.
So we:
• do not host any book files
• do not sell any books
• do not provide any book downloads
So, there's two things wrong, here. (1) They are selling you a subscription so that they may provide you links. They do not have the books - all they are doing is directing you to other sites that have the book(s). Kind of like a person on the street charging you for directness to the library.
(2) This statement (above) is usually a ploy around any copyright infringement claims. They can look all innocent and say they are not the ones providing the books, they are merely providing links to where the books are (which are usually unauthorized uploaded copies that are in violation of copyright). It's the servers that actually HAVE the books that are doing wrong, you see - not them. This way, they can skirt the letter of the law and appear legitimate.
Honestly? I believe your instincts are telling you true. This is not a legitimate site. They are in a gray hazy area and I'd be surprised if it was not copyright infringement.
We can't tell you what to do, but I know what I'd do. Run!

I certainly would not give this site any personal information (esPECially a credit card!). The deal may sound good - but I believe that's because it's a little too close to the line of legalities (and a tad over it, IMHO).
Sorry - but my recommendation is to pay the extra and get the book from a legitimate source.
Marilyn