The Kindle Paperwhite has some features that can help you become a better writer, in an enjoyable and relatively effortless way. Other versions of Kindle or other devices may have these features, but the Kindle Paperwhite is the one I'm using.
Firstly, if you want to write well, you must read widely. Any ebook reader can help you there. The classics are available free or at a very low cost, and when you hear about an interesting new book you can often obtain it immediately. Don't just read the popular works, though - and do read books from the previous decades and even the previous century, if you are not already familiar with them. By doing so, you will automatically absorb correct grammar, sentence structure, spelling and a wide vocabulary. You will see words and phrases in context, and will be less likely to use them inappropriately.
The Kindle Paperwhite has a direct link to Amazon and to Goodreads, the book reviewing application, where you can review books you have read and see recommendations for similar books.
The next useful feature of a Kindle is the dictionary. You can choose from several dictionaries, for example the Oxford or the Merriam-Webster dictionary. Sadly, the Macquarie Dictionary, which is the standard for Australia, does not seem to be available on Amazon. For Australians, the next best option is the Oxford. Once you have chosen your dictionary (under Settings), when you press on a word the definition is displayed. Since this is so much more convenient than looking the word up online, or dragging out volume 3 of your hardcopy dictionary, you are much more likely to look up words you are not familiar with and find out the exact definition, rather than relying on guesswork.
The Kindle Paperwhite also offers to look the word up on Wikipedia, which may or may not be useful.
The new feature that is offered in the Kindle Paperwhite, which didn't exist in the Kindle Touch, is the Vocabulary Builder. If you have turned on this feature, every time you look up a word in the dictionary, it is added to the list of words you're learning in the Vocabulary Builder.
When you go to Vocabulary Builder, you will see the words you've looked up, and you can see the definition. You can also see the word in the context of the sentence from the book where you read it. Even if the dictionary didn't have a definition, the context may be enough to remind you of the meaning if you looked it up elsewhere (for example, on Google). In this way, you will increase your vocabulary in a fairly painless way. More importantly, you will be reminded of the context, by seeing the word in the sentence from the book you were reading. This will help you avoid the classic mistake of using a newly learned word in an inappropriate context. If you have chosen well-written books to read, you will see new words used in interesting ways. When you are confident that you know the new word and how to use it, you can select 'Mastered' and it will disappear from your wordlist.
All of these tools can also be used to learn or reinforce your knowledge of a foreign language.
By learning new words and the appropriate context for their use, you will become a better writer. By reading widely from the classics and the best modern literature, you will learn to write grammatically correct sentences. By increasing your vocabulary, you will be able to use vibrant language and avoid using cliches and jargon. All of these benefits can be obtained in snippets of time, on the bus, at lunchtime, or in the evening. You will become a better writer, confident in your knowledge of the English language.