I believe reading is the best way to improve vocabulary, but, reading is not enough! You need to look up new words in a good dictionary like the Cambridge Advanced Learner's Dictionary, which has good sample sentences that help you understand the word better. Then, you need to keep a record in a notebook, on flashcards, or with digital flashcards you can make on so that you can review new words before you forget them! Finally, you should use the new words in your own sentences and say the sentences to a native English speaker, like me, to get feedback as to whether you are using the word correctly. Sometimes, it's easy to make mistakes when using new vocabulary in your own speech, especially when using phrasal verbs and idioms for the first time, so it's best to try it out with me before you use it on an IELTS or TOEFL test!
I like to read books. I can't imagine my life without books. Definitely reading can help everyone to improve their vocabulary. In my opinion, the most important thing is to find English books on your level. It's not easy. I recommend reading books from one author. For example, now I'm reading books by Gary Paulsen. I highly recommend this author. He is one of the best loved writers for young adults. I'm not a teenager. If you take and read books from the same author one by one, you will notice that you remember the new English words without any effort. Of course, as Janine said, we have to review and use these words.
I just remembered about for improving vocabulary. Have you tried it?
Who has has enjoyed leveled readers like the Penguin Readers? Many of the books are classic British or American novels rewritten for different levels of English proficiency. You can listen to audio as you read a novel rewritten for learners at your level. I think there are samples on YouTube.
If you don't know about leveled readers, check this out:
Black Cat and Macmillan also have leveled readers.
On this next site, you can take a level test, but it might drive you mad because it begins by checking very basic vocabulary. You probably have an idea of your level. If not, ask me, and then just check out the books available here: