How to Expand Your Vocabulary
David J Pfeiffer

Being able to tap and hold on a word to get the definition on the Kindle is enough for me to stay away from physical books alone. The flashcards are incredibly useful. It has helped me a lot when reading books in American English, where you see a lot of words British English speakers do not come across.

I would also suggest a further tool to memorise words: Type in the word and it will produce a nice web of similar words, helping you remember new words quicker.