Of course I remembered more books. I will use this current entry for anything else that occurs to me, so come and check it again in future.
First, there are books on legal English by Peter Tiersma. The one I know is called Legal Language. It’s a good read and need not be read from cover to cover. There’s also one called Speaking of Crime, which I don’t know. Plenty of information on Peter’s website about these.
Then there are books on legal style, for instance those by Mellinkoff, which are particularly good. These can be particularly helpful for translators out of English, for instance if they want to know if a doublet (such as aid and abet, let or hindrance, cease and desist) contains two ideas or one.
Thirdly, I had forgotten that the University of Cambridge is introducing a legal English qualification with course materials. This is a rather ambitious project. I did investigate the materials in a previous entry. Here’s a link for further investigation, and here’s one to the book. The book can be bought now for £26.95 – student’s book with three audio CDs, suitable for class work or self-study. It looks fun, but I notice it doesn’t distinguish between British and American usage all the time – the author is probably American.