Do you need a book light if you have a Kindle?

No — every current Kindle has a built-in, adjustable front light, so you can read in a dark room without any extra accessory. A clip-on light like the Glocusent Book Light is really meant for print books and hardcovers, which have no light of their own. So if you read on a Kindle Paperwhite you are covered, but a book light is still worth having for physical books and bedside reading.