Check out the training modules here for smarter, better, faster writing. They include:
Words a waiting modules (5 minute reads for an immediate knowledge boost) and
Words at work (half hour reads that end in an exercise for you to practise your learning).
Both sections contain training on grammar, expression, sentence construction, tone, chapter development, document structure, types of work documents (including correspondence) and plenty more. Each module has a number and a title, so you know exactly what you’re getting and where you’re up to in the whole program. Feel free to skip modules that you already know, and ask for modules that you can’t find here.