Reading books on tablets or phones is awesome. There, I said it, and I'm not taking it back. While the biggest advantage of reading on a mobile device is convenience and a huge portable library, there are a lot of features that make the experience awesome.
For simplicity's sake, we'll look at reading apps on your phone or tablet, but there are also plenty of advantages to dedicate ereaders. Not the least of which is e-ink displays that are less harsh on the eyes, and built-in book lights. That's a whole different debate we could have (maybe in the comments!), but let's look at the sweet features of the much more widely available apps for now.
Keep Track of Characters With Kindle's X-Ray
OK, so hang on. Daenarys' father is the Mad King, and she's the last Targaryen, but wait, then how is Aemon related? What's he doing on the wall? Is there anyone in King's Landing Joffrey isn't related to? Even in simple stories, keeping up with characters can be an annoyance, but big stories can be downright impossible. Which is why Amazon invented X-Ray. This feature allows you to read up on characters, ideas or notable items so you don't get lost. Just mind the spoilers. Frustratingly, the Android app lacks the feature, but it's available on Kindle and iOS devices.
Learn About Your World With Wiki and Search Integration in Google, Kindle and iBooks
X-Ray is great for sorting through which of the bajillion houses you're supposed to be rooting for, but sometimes you need basic reference information, like where the heck Fray Bentos is. Amazon, Google and Apple all have pretty great search built right in that allows you to select any word or phrase and find out more about it on the web.
Search Scanned Books in Google Play
Both Play Books and Kindle have the ability to search modern ebooks, but for older books that are made from scanned pages (like Sherlock Holmes or the Jane Austen book that's included with every account for every service ever), you've been out of luck. Until now. Google recently added the ability to search scanned pages.
Translate Text Into Your Preferred Language in Google Play
A perfect universal translator might not exist yet, but Google's works pretty well to help you get the gist of things. It also happens to be built-in to Play Books. Select a length of text and tap the globe icon on the Action Bar. Here, you can choose any language to translate the text into without leaving the app.
Buy (or Rent!) Souped Up Textbooks With Google, Kindle and iBooks
Tablets' high price tags tend to prevent them from creating the revolution in education that we'd all like to see. However, Google and Amazon both offer textbook rentals for steep discounts versus what you'd pay buying the paper copy. The downside to this is that, in general, Apple's textbooks via iBooks have a lot more features (and wider selection), but you can only purchase them. Still, if you're willing or able to pass up the tactile experience, you should probably check to see if any of the three companies carry the textbooks you need for your classes.