You seem to be laboring under a major misapprehension. The fact is, the “free book” you offer on your law firm's website doesn't have to be the kind of book that's reviewed by The New York Times, and it doesn't have to be the length of an Elmore Leonard novel, either.
When a prospective client in need of legal help lands on your site, the last thing he wants to do is spend a week reading a dense, hefty tome about the minutiae of the law; rather, he wants a quick, reassuring read that will set him on the right course for planning (and hopefully winning) his lawsuit. You don't need to write 50,000 words; one-tenth of that will do just fine, or even less, assuming the information you impart is genuinely valuable.
You also don't have to worry about printing an actual, physical, paper book, of the kind people can buy in bookstores. It's perfectly okay for your free book offer to consist of an electronic “E-book,” with minimal graphics, that a prospective client can download onto his hard drive with the touch of a button. (Of course, this transaction allows you to capture the person's email address, so you can follow up with him in due course.)
Your visitor won't be disappointed at the brevity of your book, but he will be disappointed if you fail to deliver on your promises—so if the name of your book is The Ten Mistakes That Can Sink Your Personal Injury Lawsuit, you'd better make good on that title!