I checked that out of the 430 books listed on Seferim.net which ones are listed at Google Books, LibraryThing and WorldCat. There were 386, 331, 394 respectively. I added all these links to the appropriate books.
Technical sidenote: In order to do so I developed a helper tool. It allows opening multiple (25-50-100) URLs at the same time. The users can paste all the URLs into a textbox and then can define how many of them should be opened and which one should be the first one. This allowed me to go through Google Books and the other two sites, 50 by 50 checking whether they have a certain book or not. I made a version of this tool a year ago, but that depended on a mySQL database table. Now it doesn’t have this dependency. I will tweak the tool a bit more and then release it for anybody who is interested.