When I went back to college to finish my degree, some of my classes used online textbooks from a couple different systems. Most had a simple link to a glossary for key terms, but some took it a bit farther and had a nice pop-over widget. The nice ones also had the ability for you to highlight and annotate passages for your own notes. It's less fun though, if you're like me and have a hard time reading long-form content on a laptop or phone. I ended up getting one of those eink Android tablets to make it easier for me to get through the reading.

Shame is that monetization around them is even more exploitative than normal textbooks. You don't own them, so you can't keep or resell them once you're done, and you typically lose access to it about a week after the class ends. Many courses also issue assignments and grades through the e-textbook, so you're forced to buy it at a price they decide. Fortunately work reimbursed mine.