Think it right, everything has legal risks, since anything can originate arguments that may seem as valid for somebody. That's the reason why court houses exist: to judge if an argumentation is legally valid. I know this might be a quite extreme affirmation but, yet, it's a valid one.
In the first place, you have the legal right to have and express opinion. If you read or watch something, you can make a value judgment on it. There's a difference, by the way, in giving your opinion - what's a product of personal believes - and affirmation - what's a product of a fact.
About permissions, that's why licenses and terms service are for. Check Tweeter Terms of Service!
You retain your rights to any Content you submit, post or display on or through the Services. By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).
This license is you authorizing us to make your Tweets available to the rest of the world and to let others do the same.
You can use tweets as much as you like as far you respect the terms of service. If you want to use other sources of data, you will need to check the license of that source.
Of course, you will always have the right to express your opinion on a book even if it forbids reproduction, as far as you don't use any of its content.