The reading list at the end has two of my favourite books: Butt and Castle on Drafting and Thornton on Legislative Drafting. It also has four weblinks to similar materials in Australia, Canada and the UK.
This useful link comes from a colleague (thanks, Siriol) via Daphne Perry, a lawyer who is the UK representative of Clarity. At the Clarity website, which I must have mentioned at some time in the past nearly ten years of this blog's life, you can see earlier copies of their journal. A large number of arguments for and against shall and must are given. The conclusion is that shall is used less than it used to be, and some of the current uses in legislation may be where a statute has been amended and the drafters did not want to change the usage where the original used shall.
In relation to translation: one reason to avoid shall which Daphne Perry mentioned is that she heard several Arab lawyers say they were told always to translate 'shall' into Arabic as expressing an obligation - but of course, it is used in several other ways too. (I have less of a problem translating into English and using 'shall' only to express an obligation).
Particularly interesting on the perennial debate about using 'shall' or 'must' is a 2008 paper by the Drafting Techniques Groups at parliamentary counsel, another PDF file: Shall. It discusses the non-legal use of shall too. I am not keen on the use of the term 'simple future', although I know what they mean. There is reference to the Scottish and Irish reverse the English usage of I/we shall - you/he/she/they will,
E.g. the drowning Scotsman who was left to his fate because he cried I will drown and no one shall save me!
but I was surprised to read
But the first person shall lingers on in questions like Shall I make a cup of tea?
That usage is not 'simple future', but modal. But my Northern Irish colleague would understand it as 'simple future'!
Incidentally, googling for "drafting techniques group" also brings up other papers, for instance on gender-neutral language.