Can you comfortably read Kindle books on your MBP?
Not really. Sure, it can be done but it is somewhat restrictive. I strongly recommend getting a Kindle for Kindle books - and I recommend this over an iPad for kindle books. The reason is that a laptop anchors you to the device, you can't easily read while lying in bed or other places. A kindle, like the $79 Kindle is extremely light and fulfills the requirement of being 'invisible' while a iPad is 'relatively' heavy to hold for long periods. An iPad uses reflective glass making it tricky for reading outdoors or in bright areas, and for some people, especially at night, the backlighting can get hurt your eyes - if reading for long periods.
I have a Kindle app on my 15" MBP but rarely use it. About six months ago I decided it was time to move from paper to an electronic device mostly due to the problem with eye strain and small fonts - yes, I wear glasses but small fonts can be dreadful when reading for longer stretches so I had to make sure books I wanted didn't have tiny fonts. Anyway, I had to decide between a Kindle and a iPad. Money is tight, I already have an iPhone (with the Kindle app) and a 15" MBP but neither did the trick for reading. I decided on the $79 Kindle and couldn't be happier. It is extremely light and no glare and I can and have, held it for hours. I can adjust fonts as desired. As a result, in the past 5 months I read more than any other 5 month period in my life, 15 books. The $79 kindle also have offers which do not use the reading space and can be really great, twice the Amazon offer was for FREE Money - buy a $5 Amazon certificate and get a $10 certificate. These offers help defer the cost.
So I recommend considering a real Kindle, just remember, a Kindle is not an iPad which is a powerful and versatile device. A Kindle is designed to do one thing well, being a dedicated reader. Consider it, and you can always purchase one and return it in 30 days if it is not for you.