Reading is a good habit. It increases your vocabulary but only when you take the pain to explore new unknown words by flipping dictionary pages(you may even use your smartphone to reduce the trouble if you are lazy like me).
Underlining words and writing the meaning has 2 advantages:
1) You may not remember the meaning of the word the next time you read(Usually happens with me if not written). You would not be required to check that word again in dictionary if you had already written the meaning. Whatever you write gets embedded in your memory. So it's less likely that you forget it.
2) "It is like visiting a forest where you once blazed a trail.You have the pleasure of going over the old ground and recalling both the intellectual scenery and your own earlier self" - William Phelps