It's good to remember that Google offers a feature called Google Books, where you can often find extensive previews of books on practically any topic. Since academic books (in other words, ones usually published by university presses) require "peer review," those are the best books to look for. "Peer review" means that before a book can be published, it has to be read and assessed by other experts in the field. Thus, if you look for academic books on Karl Marx, you are more likely to get information that is scholarly and reliable. Here is a good place to start:
Other than online cites, there are most likely still books in the library about Karl Marx. Check with your school or city librarian or look in the card catalog under subject: Karl Marx, and I'll bet you find something.
Or, if your state has a library network that collates scholarly articles from all over the state on one website...that may also be a place to start.
There is a lot of information about Karl Marx on the web. Since you are asking about Karl Marx under the topic of Animal Farm, I will direct you to sites that can give you Animal Farm-related information about Karl Marx and Old Major.
In addition to the ever popular Wikipedia, I have included several other links that will give you information about Karl Marx, the German philosopher and revolutionary who was an early founder and advocate of socialism and communism.