The Seasoned Schemer by Daniel P. Friedman & Matthias Felleisen

• The Eleventh Commandment: Use additional arguments when a function needs to know what other arguments to the function have been like so far.
• The Twelfth Commandment: Use (letrec …) to remove arguments that do not change for recursive applications.
• The Thirteenth Commandment: Use (letrec …) to hide and to protect functions.
• The Fourteenth Amendment: Use (letrec …) to return values abruptly and promptly.
• The Fifteenth Commandment: Use (let …) to name the values of repeated expressions in a function definition if they may be evaluated twice for one and the same use of the function. And use (let …) to name the values of expressions (without set!) that are re-evaluated every time a function is used.
• The Sixteenth Commandment: Use (set! …) only with names defined in (let …)s.
• The Seventeenth Commandment: Use (set! X …) for (let ((x …)) … ) only if there is at least one (lambda …) between it and the (let …), or if the new value for x is a function that references to x.
• The Eighteenth Commandment: Use (set! x…) only when the value that x refers to is no longer needed.
• The Nineteenth Commandment: Use (set! …) to remember valuable things between two distinct uses of a function.
• The Twentieth Commandment: When thinking about a value created with (letcc …), write down the function that is equivalent but does not forget. Then, when you use it, remember to forget.