Advice on proofreading for law firms?

I have been working as a proofreader for some time for a number of different businesses and charities, but would really love to get in to working for a law firm as a proofreader, either freelance or in-house. I just wondered whether this is possible without actually having worked within this industry before? Basically, I would like to learn about the industry first (I am particularly interested in criminal law) as I expect (as with anything) you need to have at least some knowledge first. As such, I wondered whether getting a few books and then offer my services on a voluntary basis for a while would be a good way of going about it? Ideally, I would have undertaken a course but my finances don’t stretch that far at the moment – having looked in to it I have found them rather expensive; also, I have already graduated with a degree in another subject so university is not an option!
Anyway, I would really appreciate some honest advice on whether this is worth pursuing and if I am going about it the right way?!

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