How do companies like shortsqueeze.com find out exactly how many shares short are sold for any particular stock? Do the query each broker (seems difficult to me since there are so many brokers) or is there one central location where they get the data? Do brokers have to report the number of shares shares short in their firm to an agency? Can anyone recommend a book or web site that explains in detail the process of how brokers go about borrowing shares and lending them to short sellers and how they go about returning them to their original owner when the short seller buys them back? I'd appreciate any feedback. Thanks.