UMMM... I think, no I'm almost absolutly sure it is illegal to obtain another sites database without some type of contract with written permission. Then they would give you access to what you needed, most likely. I hope I didn't misunderstand your post We just don't support hacking here.
I have just done a yahoo search to see if there were any databases online available for download and could not find a single one. So it appears that there are three ways of getting the data into your website they are:
To spend hundreds of dollars making a deal with someone who currently has such a database
To spend about a thousand dollars getting one developed by a company
To download an application (exe) that contains the data and to catalogue all of that data into your website which I believe would fall under the fair dealing act if your download has no commercial license. But it could take years cataloging every book in existence single handed.
So those are your options and if I were to choose one I would choose option 2 as you would then own full rights to the data and might even be able to resell it.
http://isbndb.com/ has an api you can use to access THEIR data. I can't see anybody giving away a database for free.
You've also got the problem of data veracity with many free sources. Some of the dbs (possibly isbndb.com included) seem to 'scrape' data from sites - some data on those sites may be unverified or just downright wrong.